How to play against Sinistar.

Well, for each faction, you can develop a strategy for playing against the Chaos faction. It's beneficial for Dwarves to use the elemental immunity rune on their main stacks, for Mages to play around magnetic golems, for Necromancers to overwhelm with hordes, for Elves to resist the aura of horns, for Humans to rush with buffs, and so on. It all depends on the players' experience.
And, of course, take skills and artifacts that increase magic resistance.