Here's a build to try beating the caster in a fairly mundane manner. It should work for any version, assuming he can somehow sneak into the demiplane of awesome.
Assumption: that Diplomacy will work on the opponent. Because this is the kind of build I'd use to kill off a BBEG, not another PC.
Plan:
0. sneak over to him with darkstalker, hide in plain sight, and anything else I can get going.
1. get an insanely high bluff check to use Wanderer's Diplomacy to bring the wizard to fanatic as a standard action.
2. Dread fang of lolth to act in a surprise round.
3. legacy weapon for cunning ability - never flat-footed.
4. get comfy with the wizard
5. use poisons and stab him in the back repeatedly.
How it should go down:
1. Sneak over to the wizard. With dark creature and Darkstalker, along with max ranks in hide and move silently, odds are good that it will be easy to get close to the caster. At some point Foresight will trigger, starting initiative. The character is never flat-footed thanks to his legacy weapon, and always gets a full action in a surprise round thanks to the capstone of dread fang. Even if the legacy weapon doesn't work, he still gets a standard action in a surprise round, enough to do his thing.
Even without the legacy item the character still gets a standard action on a surprise round - enough to use Bluff as a standard action to convince the wizard to be his fanatical follower for the next minute. His bluff check should be incredibly high, with motivate charisma from marshal and about 2/3rds of his WBL spent on boosting the check. The rest of WBL goes into making some ridiculously deadly poisons, a belt of battle, masterwork handcuffs, and some boosts to hide/move silently.
Then he talks the wizard into getting naked for bondage sex, binds him, gags him, and stabs him full of colossal spider venom.
Build:
Dark Half-Drow Rogue 4/Barbarian 2/Dread Fang of Lolth 10/Exemplar 2/Marshal 1
Class variants:
Barbarian: trades rage for favored enemy (UA). Trades fast movement for pounce (CC) and uncanny dodge for improved trip (UA) as well - because I can.
Feats:
1st: Skill Focus (Bluff)
3rd: Darkstalker
6th: Least Legacy
9th: Wanderer's Diplomacy
12th: Lesser Legacy
15th: Open
18th: Open
Put skills into: hide, move silently, craft (poison), bluff, diplomacy, UMD.
How it comes together: rogue fulfills the main prereqs for dread fang, while barbarian gets the BAB +5 done. Exemplar is used to get skill mastery in all of these skills, and to rebuild bluff ranks after the levels in Dread Fang and Barbarian. Marshal is for motivate charisma and gets an exp penalty, but is taken at 20th, so who cares?
Favored enemy: Animals. Take THAT, Dire tortoise!
Equipment: a weapon with the Cunning ability, plus poisons. Hell, Bracers of AMF might even work out, though then it triggers crazy hat contingencies and stuff, so why bother?