How to play against Sinistar.

I would still recommend investing in Magic Resistance (in case it triggers, the damage is reduced by half on the Unyielding difficulty). You should focus on building up your army and leveling up the Light skill. If possible, try to prolong the game, find Shackles, and you can also equip artifacts that boost resistances (especially if you know your opponent's preferred element). That's how I would approach it.