[VonDondu]

There is no defense against Time Stop per se. Only three creatures in the game are immune to it, and you can't make your own characters immune to it without cheating. On the other hand, there are ways to limit the damage. The best way is not to let the enemy cast it. There are many ways to disrupt spells, except in cases where the game does a scripting thing called "forced casting". Killing your enemies before they can cast any spells works great if you can pull it off (by using Time Stop yourself against them, for instance). The second way is not to be within sight of the enemy when they cast Time Stop. The cheesiest way is to run to another room when the enemy starts casting Time Stop so the spell will be wasted, but you can also use summoned monsters to draw enemy fire so your enemy's spells will be depleted before you send in your main character. Just watch out for the Death spell, which will unsummon your summoned monsters (a common hazard in Throne of Bhaal). I can't remember which summoned monsters are not affected by the Death spell, but there are a few. In general, it's a good idea to keep your party members spread out, anyway, so that area affect spells cast against one party member, for instance, will not affect all of your other party members too. I have my party "tag team" some of the more powerful monsters.

By the way, I don't remember there being any Mages on the first map who can cast Time Stop. Which map are you referring to, and have you installed any mods?