Why Medieval Era?

I mean... you could go slice-of-life then ULTRA MEGA WAR  forced into plot. 

*reads 'Area 51' memes*

God:  

See?! EVEN MY SQUAD COMMANDER AGREES! Lest us be an enternal disgrace to thy own creations! 

IIRC, shields were being used less not because of ineffectiveness but because weapons were getting more and more 2 handed so they ran out of hands to hold weapons AND shields and decided to go with the weapon instead. 

There is a reason why most stories don't have a scenario about gatling gun and being surrounded at the same time. Cause if the MC can deflect more than 10k bullets per minute, why not just give them immune from ranged attack skill?

Also when you give MC such OP skill it will be very hard to write a good story without going into the realm of 1 million sword slashes in 0.1sec

Note: gattling guns(miniguns) shoot at least 2k bullets per minute according to wiki. 

Liam trained for decades and has bullshit sword skills and reflexes that he can cut steel meters away while looking like the sword is still in his sheath, he also has a magic mecha with a shit ton of guns stored with spatial magic. 

Eh, often it's not even medieval, but a hodgepodge of historical era of Europe 

Well fantasy genre are based from folklore that are constructed with the lore of kings, dragons, knights, and gods.