Y'know, considering it is a giant undead spider, it never occurred to me to check if it's basic attacks dealt poison. That'd make stoneskin less important by far.

My problem with the first islands is that the game is less interesting when you have very few options at hand. It feels like a grind after a dozen battles... later on, one has far greater variety in spells/abilities, but unfortunately many of the combinations are so OP that it becomes too easy... (or the opponents are too weak / need special abilities as well)

About the spider: I thought only the ranged retaliation spit deals poison damage? Stone skin seemed to work well on melee attacks. And my 3 trolls (in one stack, which shouldn't matter) survived the attacks without a problem, not sure why they die in Avo's case. Are a lot of the small spiders attacking them as well? (those deal poison damage)