Monster res are the same for 1.10+ versions I believe. They gain bonuses to res from their other mods: fire/cold/lightning enchanted, magic resistant, stone skin, mana burn, spectral hit...
It would be a magic wand that you could shop (reroll the shop by exiting and returning to town). It would be "blahblah of lower resist", where blahblah can be any wand. I'd recommend trying Ormus, Jamella (?), Malah, Drognan or Akara for rerolling. whichever is faster. You'll need a few hundred thousand gold in case the wand has some other expensive skills on it like lvl 30 skills (poison nova, etc.)
Act 3 mercs do VERY little dmg and are altogether fairly useless. Buy an A2 or A5 mercenary for better results. A2 Offensive merc from nightmare difficulty will have might aura, which will boost his dmg considerably. Then you can use any high dmg polearm or spear and he should be able to kill most things you can't, with some help.
Poison dmg would come on your merc's weapon or helm ideally, but it's not necessary. It could also work perhaps if you had a bow on switch but I realize your chance to hit would probably be too low. Telekinesis is a better use of your time facing enemies you cannot kill without the merc. Aim for as much dmg as possible on the merc and then outfit him with life leech/crushing blow/resistance if you can. resistance on the merc >> defense, so make res a priority