^This.
On the vid, the fight looks extremely tough because the guy is running a sustain build (without reaching -50% atk) + an off element KH, with only 12% def down trough eidolon, and no element advantage. His damage output is really low (its pretty low even according to Nutaku's actual standards).
Sanahtlig is talking about ppl squeezing the fight, but most of the time they are just overkilling the boss (Fire team with Shingen and x2 Bellial one shooting it without Mars' debuff, Light team with Nike going trough the rage phase without even using his 2 full burst, Water team with water Osiris not using his burst just to lol at the boss overdrive doing 0 damage, all of them without element advantage)
Just tip アクセ5層 on youtube and see by yourself
I wouldn't worry too much honestly. If your team is (or your teams are) properly levelled (and optimized if you're running a main team without element advantage) there is no reason to not clear it in the end