fire isnt as bad as poison. fire is countered by shields so you can mitigate the damage assuming you have adequate shielding by the time you reach the third level. Honestly if you can't deal with the fire geckos by either overpowering them or have the health pool and shielding needed to survive then you're probably not gonna beat the final boss either with whatever build you have.
Poison definitely is a run killer at times, though. I don't know if it really needs a nerf but it would be nice to have more options on cleansing it because you either are lucky enough to have the rag and weapons to trigger it or just keep a couple potions on you that clear your status effects. I've never made it past the snakes on a second run because they stack poison so highly and have such huge health pools. Something to reduce fire would also be nice, even if its less dangerous since it can be blocked.
the biggest problem for me was the fact that they limit the possible builds so much... i tried a no weapon run and snakes and bat were my downfall every time, on my successful attempt i had to get regen that healed all my health in 1 turn because if i missed even a little bit of hp the poison would one-shot me... definetely op
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".