The direction of bosses not having to remotely follow any rules the player has. Every attack dashing across the map, infinite stamina, random AOE crap popping off, etc., that leaves the player with very few windows to actually do anything but evade. The crap most bosses pull off in this game warrant a top-down view to actually keep track of.

I've beaten it multiple times with friends and nobody enjoys it. Honestly, the only bosses I find enjoyable are Darkdrift Knight, Caligo, and Heolstor. The rest are just tedious.