I would extrapolate on the Mage-slayer build I've been plotting for a while now, but I know the DM I intend to run him under frequents these threads.

That said, pierce magical protection, pierce magical concealment. You've got one stab. Make it count.

Add in perhaps, Insightful Strike, and well...isn't there a Luck spell that counts your next roll as a nat 20?

Poisons are nice, but you want something to cripple casting, and keep him from moving.