ok i agree that was quite a quick end for the level 4, the author could have come up with another chapter of fight before finishing off it.
however, let's not forget that the level 4 had already received some damage from allen and lenalee in the last chapter and it seemed so undefeatable previously because as Faust said, the generals fell into the level 4's trap by surprise.
moreover, the level 4 only attacked the weakened black order after lulubell's surprise attack. besides that, lavi, kanda, lenalee couldn't fight because of their damaged/unrecovered innocence, krory still unconscious. allen and bookman were also damaged from the fight with lulubell and level 3s...
in my opinion, the order was in a disadvantageous situation since the beginning of this arc. actually, i am happy that this HQ attack arc is almost coming to an end (i suppose). time to move on...