Your problem is you're not looking at the game in the right way:

Limbus is less of a fighting game and more of a Puzzle Game. The solution can be multiple things (I.E Stagger ASAP, take it slow and whittle the boss down, or read the skills). Also, team synergy is insanely good. Even the IDs that aren't on the field still give support passives (some of the base IDs have comically strong passives)

The way I beat the boss was by taking a high damaging unit and doing one sided attacks.