For help with Acid Rain, poison resistance is determined by your Constitution, and can be boosted (for one character) with crystals. Hammer does bone damage; I don't know if anything in particular helps against the stun effect.
I don't recall having any particular difficulty with witches. If you can not get your defenses up enough (try another area first to level, if possible, check merchants for equipment upgrades, etc), what about your offense? Switch to power arrows rather than regular arrows, etc. If you keep the act 1 skeleton doll out of melee combat, it can provide good support with a crossbow (set to aggressive).
