One of the best anti-casters I personally have played is the ranger AA. Casters tend to stay back out of melee range due to their weaker ac and hps. What best to hit them with then arrows or another caster? The ranger AA can take casters down with relative ease while the melees do what they do best. Due to the fact that the ranger can have both elemental resistance and protection, fom, not to mention a high reflex due to bieng mainly dex based he is able to withstand many spell effects. Make him a drow and you get spell resistance added. Also don't forget the ranger does get some self healing capabilities to help him out. Last but not least is the different arrow types available to the AA the best of which being the slayer arrows at level 18. In my opinion this makes this character one of the best anti-caster classes in the game.