I fought this one two different ways. First time I lured first troll out and did him in with everything, including fire arrows.

Then hasted party and sent mages in with a few tanks. Tanks went straight for the badguys and the mages started casting spells.

Tough fight, but he did fall.

Second time was so much easier!

You can come up the stairs immediately behind him. I did it by accident. My party went out to open the drawbridge on first level, had to climb up to second level of roof. So we started clearing the roof, and entered second floor from there. Bang! Right into the mage. Backstabs/lilacor attacks and mages were back too far to be hit so they sent spells (acid etc) at the troll. The badguy mage is btwn your party and his pal troll.

I've gotten into the stronghold and up some stairs. Now I'm facing a mage that's in the same room as a troll and ice-troll. The ice-troll I managed to trick to go out of the room to be killed. But the troll stays.
Now, I can't manage to kill the mage, no matter what I try. Anyone has a tactic to kill that mage?

If I remember, he puts up some protections and then turns invisible. Just sneak into the room and backstab. Or walk in and when he turns on all of the enchantments, run out of the room, wait for everything to wear off, and go back in fully prepped (haste, potions of strength, etc.).

My tactic on this one (I am assuming you mean the Yaun-ti mage in the library looking room). I had my pc (fighter) and Minsc attack the two trolls (the ice troll is easy to kill) and had my other four (yoshimo, nalia, airie, and jaheira) blast the mage with arrows and bullets (magical, b/c I think the mage casts protection from normal missiles). This seemed to work ok for me. Minsc and my fighter took a little damage from the troll, but the worst the mage cast on me was confusion (oh no!). Hope this helps a little.