I have been struggling with demololisher insta kill and I would keep trying to punch the demolisher but he keeps pushing me etc any tips

Sorry for the late response, but if you're talking about the one that shows up in The Pit, for Any% runs we level our unarmed damage from the Legend skill tree to max so we can melt it in like 1-2 hits. Though you also need to be on Story Mode so the demolisher is even weaker.

If that's still not working, then I don't know, unless you're doing NG+ runs where it has more health? It should still go down relatively fast though.

Dodge his attacks and stay near him so he doesn't use the ram attack