Tanks in general don't work at harder difficulties, everything hastes itself and deals a ton of damage in melee. Just summon up a bunch of guys to take hits if you can't nuke everything down in a turn or two. I just finished a duo on torment (fighter + mage/priest), and the fighter wasn't able to tank until I got divine warrior, even with blessed plate and a shield he got his ass kicked.
I don't think Int increases spell target numbers, learning the spell at higher levels does that. In any case you can get the top level spells right after getting to the Vahnatai capital when you teleport to the Tower, so you should try to have 16 by then for divine warrior.
Most low-level magic resistant guys are only to one type. Imps I think take full lightning damage. Araneas take fire damage, but they can be tough early, I generally relied on rabid bats and asps until Fireblast. And chain web works wonders, cast it every turn and they won't even fight back - this is a good way to take out Drakes early on.
If you're having trouble in the ruined fort, don't forget that you can use repel spirit on the undead vahnatai (vahnavoi) along with the undead. Hasting your priest works well here: you can haste, wait, then walk into a room and have four casts before they get a single move, that should kill everything besides specters and your fighters can mop up.