Stacking red is pointless imo. Just stack your best non reds vs him.
You aren’t trying to kill Kashrek quickly, Kashrek has no offense and isn’t punishing at all. Kashrek is the least threatening hero on the opponents team. Kill him last. Or whenever convenient.
Scarecrow has it right.
Bring your best heroes (preferably non red). Use Kashrek as a sponge and send as many tiles at him as you can. Charge up 3 of your heroes and use them to take out the biggest threat on the opponent’s team (hint, not Kashrek). Rinse and repeat.
As scarecrow says he’s actually a pretty bad tank once you realize he’s a safe place to charge up your heroes. The way he punishes you is only if you stack reds. If you don’t stack any reds he has zero punish potential.