i managed to beat the final boss one time, but not the grand master, i doubt i would have killed him too, mf did 12 burn two times in a row, i think he does 10 or 12 burn, if he does 2 times in a row you get 20/24, and grandmaster summon other fire geckos, so if you dont got a build with enough damage to kill a fire gecko instantly, the fire is going to melt you.
The poison and fire should just be recuded in half, it would still be dangerous if you dont kill them fast enough, but not like "omg i need to kill him right now to have a chance".