Marty, I've never been able to dispel their protections. The best strategy for dealing with their protections is just extending the battle long enough to let them run out.
This battle is an excellent example of how the proper strategy will make a battle seem easy.
Here's my strategy.
1. FULL buffing before talking to the person to takes you to I&M.
2. After talking to I&M, run *AWAY* from them immediately. Madae casts a Blasphemy spell and it has a limited range. Running away can get some or all of your party out of range.
3. Send in 1-3 of your party to take out the summoning mage that is in the room to your right. Kill him *fast* and you won't have to face those mobs of enemy summons.
4. Have your own spellcaster(s) summon up a mob of your own and send them against I&M to keep them occupied, while your party takes on every other enemy in the room.
5. Keep on summoning monsters as I&M kill them off. Whatever you do, keep I&M fighting summons and not your party.
6. When you start to see one of the twins take damage, throw everything *and* the kitchen sink at one of the twins. (Isair seems to weaker of the two to me.)
7. FYI... If I or M has a fireshield up, do **NOT** attack I or M with a Mordenkainen's Sword. While Mord's Sword may seem like a ranged attack, the game treats it like a melee attack and your mage could end up killing him or herself before you realize it. (This happened to me.)
8. After I&M run from the room (ending the first half of the final battle), quickly heal any wounds and rebuff as necessary. Without exiting the room, order your remaining summons over to the room where I&M teleported to. They'll come in handy.
When ready, exit the room.
9. At this point, my favorite tactic is to break out some Holdfast arrows to tie up the very dangerous Slayer Knights of Xvim, while most of my party hacks up the badly wounded twin (usually Isair). Never ignore those slayer knights, even while you've got them held. The holdfast arrows only last for a short time, but they're very, very useful against the SKoX.
There's no doubt that I&M are a tough pair. But good tactics can take them down surprisingly easy.