My way of clearing was... Strange
Arumat, bayley x2, nia (L), osiris and Andaria friend
Strat? If you use bb, sbb and ubb in the same turn (no ea) you will stop completely the heal
Unit killed? Bayley, the boss doesn't use a lot of attacks and stop instakilling units
Damage immune, use bb, is the only way to damage him (lasts 2 turns, every 20% hp) but still, use 2 units to use ubb and sbb to stop the heal
If you ask me for a role of each unit, almost all nuke, arumat was there for od filling and used andaria ubb to deal damage and help the team (since arumat has that extra skill), also, just nia dbb