How to deal with this GM?

[Mastikator]

On the top of my head here are some options.

1) Tell him he's a jerk for dismissing your opinion. Telling people they're jerks is usually pretty hard for both parties and may spell the end of the campaign.

2) Repeatedly TPK. Combat too hard and he refuses to turn it down? You conspire with the group to play suboptimally and all die. Can't have a campaign if everyone is dead and that's on the DM. This may also just straight up end the campaign, especially if he finds out.

3) Start a group with someone else as a DM, invite your DM and tell him "this is how it's done", if he ever complains then make an impossible encounter to kill his PC and say "do you prefer it this way?"

4) Tell him you're not having a good time and leave. This one has the benefit of cutting to the chase, probably having the best chance to actually change his mind. But also highlights the reality that he probably won't change his DMing style. You should probably ignore 1&2 and just go for this one. But 3 is always a good backup plan.