How to play against Sinistar.

Mages are probably better. They have a lot of magic immunity, although Warlocks can partially bypass it. But there are also Golems that absorb damage, heal, and have Mirror.
Demons are also good, but only if there are enough Imps.
Dwarves also have a rune of elemental invulnerability, so even if it reduces half the damage, there's already enough resistance to fire with immunity.
And an Orc with anti-chaos can also try.