Nankoufuraku no Maoujou e Youkouso

Last part of Vol.3!

Spoiler: Final battleSo Scaha tried to capture Lucy using lightning. Too bad for him he got attacked and died. Lucy left.
Benou and Wraith kept fighting. Benou kept on pestering Wraith to makr a contract with his spirit. Wraith refuses at first, but then Benou points out Fran who was missing an arm but still kept on fighting for Wraith. She dies btw from Ronowe and Orobus. Wraith immediately feels bad and now finally understands of how stupid his actions are (because even though he has the power, he chooses not to use it). So he calls out to Undine (water spirit's name) who was hanging out to watch the whole battle. He says sorry and then asks to contract with him. Undine agrees and contracts with him. Wraith and Benou then go all out and use assimilation (their body is literally made of their element now (Blue flame for Benou, Water for Wraith)). Anyways, Wraith lost the fight. He thanks MC and Phoenix in his mind (He realized their identities ^^).
Back to MC and Aerial; so Aerial made the heavenly garden thingy and MC tries to decipher it by using the Arlaunes and and himself to look for the right blocks. He uses Forus as his shield. Anyway, MC got through and Aerial applauds him because he was the first ever to defeat his tecunique. So, Aerial and MC decide to take the battle more seriously. MC releases his horn and Aerial uses the very last of his magical powers to fight. Ugh I can't remember much of the details, but in the end, MC managed to punch Aerial in his gut/chest and blasted him off the wall. (Cool right??)
So Helvor and Berito are still throwing hands with each other. After MC managed to throw off Aerial, he asks everyone out to help him. Berito was busy fighting Helvor so MC lent her a hand. He gave a significant amount of magical power from his horn to her and she managed to defeat Helvor. Unfortunately, she got hurt too badly and her avatar died.
Can't remember much of the others but I think Eri and her team fought against Yuan?? (Don't quote me on this). And the others pretty much died??
Anyway, only MC and Benou versus Aerial were left.
Aerial had already lost all of his magical powers and his magical organs were already broken because of MC's punch. So, he asks his wind spirit (name is Sylpheed) for help. The wind spirit shows up and tremendous amounts of magical power was released in the area. Aerial says that this move has never been seen by others before because the spirit only agrees only if it's really needed, and that this move was specially for Lucifer but he never got the chance to show it to him.
Benou offers MC some assistance by covering him with blue flames. MC alao releases the last of his magical power through his horn. Benou and MC go all out with their powers and Aerial doing the same. In the end, the ones who remained were Benou and MC. Aerial's avatar dissipated and the hero team finally lost.
YAAYY DEMON STAFF WINS WOOOO!!
Spoiler: AftermathAfter the battle, Wraith called out MC again. MC met with Wraith and Fran in a bar/restaurant place. Wraith was feeding Fran. Wraith thanks MC and says that he finally understands what he needs to do. Fran also thanks MC because he was able to get through him while she couldn't. Wraith then asks MC to join his team again. MC denies the request and says that he is happy with his current job. Wraith is all like "awww too bad~" but still insists on MC anyway.
After the battle, All hero team participants of the raid battle were asked to do an interview. The 5th hero team's Suri finally decides to reveal his identity to the world. His teammates cheer him on. The 3rd hero party is all happy and such. Wraith , with Fran, tells to the people watching that he is looking 2 more members for his party and that he doesn't care what the race and class is. He says he only needs 2 more members because the last one is already taken (implying that he'll keep on pestering MC to join his party). And finally, the 1st hero team's Aerial said that he plans on challenging again in the future because he doesn't plan on retiring soon.
YEAH THAT'S PRETTY MUCH IT :I
That's all ^^;
Sorry for the almost month long reply ^^;
Still haven't read Vol. 4 so I hope someone else can provide spoilers >.< 

Honestly, volume 2 felt like a filler arc you'd see in long run anime series haha. It would highlight some issues that the world has like you said about Nicola's party struggling. However, yeah the tournament fight gets boring since they focus on a lot of vanguards. I would say the spectators were entertained this round b/c Reme is known as a black mage/useless so seeing him kick butt was like an overpowered solo.
Volume 3 is where it picks up again and is actually important. 

Credits to some rough MTL translations/spoilers of @PhoenixStriker5251 at MD
MC is WAY more powerful than he appears here.
(Powerful as in a melee-battle mage that can freely resummon the other floor bosses at will as well as take on that asshole swordsman himself 1v1)
MC has the ability to turn into a one-horned devil, as a result of something Lucifer did.
In the war brought about in the previous Demon Lord's reign, the Demon Lord (Lucifer) used dark magics so powerful that he annihilated top adventurer parties single-handedly. (Heck, a veteran member of the Rank 1 party straight up invites Rene to join them when he came near Rene and felt a similar sensation to back then.)
It is implied that the government and media are suppressing knowledge about the true strength of dark magic.

The Flame hero (Mc's best friend) was bullied in the past for his dream of becoming a hero, but MC continued to support him even when he (the friend) eventually did get the hero class.
The demon girl in this chapter is presumably either the "demon of love" or the vampire queen, two of MC's future comrades.
As a demon, Rene goes by the name of "Remegeton" or something like that.
After he joins the demons, Rene lives a double life as "Remegeton" (the 10th floor boss and dungeon master) as well as himself, gathering info about upcoming challengers.

The effects of MC's absence in the party quickly becomes apparent, as the other party members seemingly lose proficiency and the viewers wondering if they were "sick" or something.
Only the Flame Hero performs somewhat optimally, due to him being the only to really understand Rene's capabilities (and thus knowing what to expect now that he's gone).

MC does eventually fight his former team. He breaks standard dungeon conventions by resummoning all (or most) of the other floor bosses simultaneously, and he combines his magic with strategic commands to capitalize on his old team's weaknesses.
Ex: He f*cking baited that a-hole swordsman into using his extending sword, used his enhanced strength to rip the sword straight out of his hands, and then impales the guy with his own sword.

Hoping someone verifies the info here 

I'm back with more spoilers from reading a few chapters again. This is chapter 28 of the novel. Lmk if you want more spoilers and I can definitely read up more and post~

Spoiler: Novel spoilersTakes so many chapters for this dumb trio to realize why their performance sucks. It's their new member (Bella) who questions if they really just think they're sick lol

-Lily and Luck are fully aware of their flaws after their performance has fallen so bad. However they are still lost as to why their performance has fallen. Alvar (a-hole swordsman) is still a dumb rock. They also basically admit that they don't know how black magic really works, thus why they thought MC wasn't doing anything.

-The team also admits that Phoenix wouldn't just keep a bad party member just b/c they were childhood friends. However, they begin to wonder why MC didn't just tell them about his abilities. Why did he just stay silent whenever they belittled him? Bella makes another good point that perhaps MC did not say anything b/c he had a good reason to avoid the spotlight.

-Bella points out that their performance has only fallen after MC left. She is the one explains that, despite the three being top-tier adventurers, MC is the one who had ability to raise their abilities even further making them (basically) super-tier adventurers.

-Lily is the only one who has understood Bella's intentions. They finally realize that with MC gone their bad performance is their original/actual ability. He was boosting them w/ his black magic. They have to fix their evaluation of themselves to improve. Finally realizing that they actually suck.

-Lily asks if Phoenix knew about MC's true abilities. Phoenix mentions that he's told them MULTIPLE TIMES that MC was more than they thought him to be and that he has never once lied about MC's abilities. Again, Luck ask why did MC hide it and stay silent when he was being kicked out. Phoenix says that isn't his place to answer / won't tell what MC didn't tell. It probably tied to how MC's teacher was Lucifer himself.

-Phoenix wonders that if MC's teacher (Lucifer) hadn't approved of MC telling him who his mentor was, would he have noticed MC's ability without this knowledge? 

Woahh thank you for the spoiler!! 

Will they drop him from the team after that? Since there's no point having 3 vanguards in the team and he's clearly the weakest of the 3 and the one who chased away their debuffer. 

So! Plot spoilers for Vol.3!

Spoiler: Before the raid battleSo after the tag tournament and beginners dungeon mess back in Vol.2, MC is hanging out at home eating breakfast with not-official waifu Mira and watching the morning news. There's been news of a raid battle that Fero organized with the participation of the 1st, 3rd, and 5th hero parties of the world, with special participation of the newly known Lake/Water hero "Wraith" and his childhood friend "Fran". MC then goes to maoujo for work.
Few days later, MC got invited again by Aerial (#1 hero, Storm/Wind hero). He went to the bar and, to his shock, Aerial wasn't the only there to meet with him. Along with Aerial, most of the participants of the raid battle were there. Aerial introduced MC to everyone. Wraith went to MC and asked him to join his party since there's only 2 of them right now, and also because Wraith is also a dungeon nerd and understands MC's value as a black mage. Aerial tells Wraith to not bug MC so much since he also wants him. Helvor (#3 hero, Hero of the magic sword) also butts in and says that she also wants MC to join her party if ever there's an opening. So yeah, the 1st & 3rd hero party, including Wraith, wants MC to join their party (woaaahh). MC refuses and tells them that he already has a job. Wraith still persists tho, but MC left the bar in the end with thoughts of preparation for the raid battle.
Spoiler: In preparation for the battleSince the people back at the bar were gonna his enemies in the raid battle, he has been piling up on research and more contracts and to gain for comrades to fight with him in the battle.
He realizes that he has lost a lot of magical power since the battle with Phoenix in Vol.1 and he used a significant amount on his tag battle with Berito/Nikola in Vol.2. So understands that he can't possibly summon everyone and help them at the same time witg his lack of power, so he proposes to the demon staff to start hiring some demons and start rebuilding their respective layers. Lucy (current demon king) approves and the floor bosses start either changing their layer battles, rebuilding their layer, or hiring new subordinates for their battle.
This is especially needed since the system of a party doesn't work like a raid battle. A normal party can keep on continuing to challenge the layer until they capture it and their party members will stay the same in the next layer they challenge. A raid battle on the other hand is a collaboration of many parties and their goal is to capture the layer while doing their best to stay alive. If one dies in a raud battle, they cannot participate in the next layers. Of course, there are some things to help when does die. For every two layers captured, the adventurers get one revive only for one person. They can also use healing potions at any time during the raid battle.
So! The following changes were done in each layer:
1st layer: Helldog (boss) asked for MC's advice and MC conjured up a plan. Helldog uses several black youth? Or some other black creature that Remegeton uses for extra subordinates. (More details on the actual battle).
2nd layer: Nothing much at least not that I can remember
3rd layer: Calmira hires several new vampires. She also hires a subboss names "Hagenti". Hagenti is a masochist and Calmira uses a whip to train her subordinates. Hagenti likes when she gets punished by Calmira. She is a bit ashamed when Reme sees her "training" her new recruits. Her layer structure is pretty much the same, but she changed a little something at the end part of her battle. She wanted to be equals/rivals with Remegeton so she was giving it her all in the battle. She asks MC for a reward (a date) after she defeats the heroes.
4th layer: Nothing much
5th layer: Sitley (boss) noticed how serious Calmira was in her battle, so she asked advice from MC. Her style is being cute because she likes cute things, but since she also wanted to be serious in her battle, she chose to let go of her cutesy style for a bit. She chooses to transform into Remegeton in her battle.
6th layer: MC still hasn't made a contract with Wepar (boss), so he goes together with Mira to her layer (Mira goes with him because she's jealous of women making a contract with MC). Forneus (subboss) leads them to her. Mira lures her out by giving uncut videos of dungeon captures. Wepar is a monster nerd and likes to observe monsters when they fight. Mc asks Wepar to make a  contract with him. She asks what's in it for her. He says that it would be a shame that she be the only one that didn't participate in the 10th layer battle because she would miss out on all of the fun together with her fellow demon staff. Wepar then agrees to contract with him. Wepar also decides not to change much of the layer, also decides not to hire any new workers.
7th layer: Nothing much, at least not that I can remember ^^;
8th layer: Not much changes in Furucas' (boss) layer. She does try her best to win tho because she heard that Mira is asking a reward from MC because of her hard work. MC tells her that he will go with her to a cafe near the castle to eat. Furucas is motivated, but MC is worried that he might lose all of his money because of how much she's gonna eat there.
9th layer: Agareth (boss) asks MC for advice because he believes that he can't win. MC says that the priority is to lessen the amount of adventurers while they fight again in the 10th layer. Agareth recruits some majin (mages/magic users) to be his subotdinates in thr battle. He also goes all out with his spacetime magic in the battle.
10th layer: MC decides to recruits subordinates so that he can lessen the burden of using too much magic/losing a of it while still looking for a way to absorb back his lost magical power. He puts out a recruitment sign asking for people regardless of race who are willing to "defeat the hero". MC manages to recruit 3 subordinates: a Lamia (has petrifying to stone powers), a unicorn (can chabge between human and horse form, is also a traiber that can control plant creatures called Arlaunes), and a minotaur who is also a black mage. MC also temporarily recruits the 95th hero party: Erie/Ellie (bold hero, can use wind magic), 2 black mages to debuff her enemies, and 2 white mages to support her. MC also contracts with his friends: Phoenix (Bennou); Lily (Strass), Nikola (Berito), Thor (Ronowe) and Orobus. He also changed his layer from one empty floor to a layer filled with rooms. Each room is a combination of 2 or 3 previous layers (like 1st room is the Hell and necromancer layer, 2nd room is vampire, werewolf and muma layer, 3rd room is aquatic and winged demon layer, and 4th room is the knight demon and spacetime layer). There are 5 rooms in all, the first 4 rooms were rebuilt to suit the past layers, and the 5th room is basically the og 10th layer that has a throne at the end with him sitting on it. The ony condition of his layer is that only the ones who can get past the rooms filled with summoned demons and go to the last room can fight him.
Spoiler: Reme finds a way to replenish his magic powersLucy tells MC that there is a way that can help him replenish his magical powers fast. She asks MC to go with her in the deepest of the dungeon. There he finds in the middle of the room is big dungeon core. A dungeon can only function with a core. Without it, the place is pretty much without no magical powers. The core acts as a thibg that absorbs the magical power in the surroundings and spreading it everywhere in the dungeon so that demons and adventurera can freely use magic in the dungeon.
Basically, since it keeps on absorbing the magic that's naturally in the air, it never runs out of magical power and has a lot of it. Lucy tells MC to absorb the magical power from the dungeon core. MC says that it's impossible because magical power, if released into the air, naturally dissipates can't be absorbed. Lucy tells him that it is possible because her grandfather (Lucifer, MC's master) was able to do it and was able transfer magical power from the dungeon core into his horns. Lucy believes that he can also do it just like how Lucifer did because he inherited his horn. Lucy lends MC the room so that he can train for the magic absorption.
MC goes to the dungeon core several times and tries to take the magical power and transfer it to his horn. He fails because everytime he does take the magical power, it would be lost and not flow towards his horn. The training is hard but he does not give up since he's had it worse when he still trainibg ti inherit Lucifer's horn when he was young.
MC wonders why his master didn't teach him this. He thinks that maybe because he was still immature back then or that it would be too dangerous for him. Mira tells him that maybe his master didn't tell teach that power because he believed that MC would be able to find it on his own without his help. MC smiles and says that he would be happy if that was true.
MC takes a breather and goes to pick up Cashew from her house and eats his second breakfast there. While he eats, Cashew's younger brother (name is Natsu) doesn't want to eat his veggies because he dislikes them. Also Natsu doesn't like MC very much and is cautious of him. MC tells him that if he loses to veggies that easily, he will take his sister away. The younger brother doesn't want that so he eats the veggies he dislikes anyways. MC says that food that is eaten becomes a part of the body. MC then gets a click from what he said and stands up from the table. He and Cashew then go ti the Maoujo.
He explains that in order for the magical power outside his body to become his own, one must be aware of it (the explanation gets harder to understand here ;; ). He goes to the dungeon core again and tries to absorb the magical power again. He keeps on saying like "this is my power, this is my power" over and over again until he was able to sucessfully absorb the power and transfer it to his horn.
He was able to succesfully replenish his magical powers! 

Thanks for the spoilers. Can you spoil on how the a-hole swordsman get his comeuppance?