|The Bald Man|
The winning condition is "You must wish the opponent dead" correct? How rule literal is this world you're in? I don't know you're gm, but thinking outside the box all you might have to do is say "I wish he was dead." Not cast it, just say it. I highly doubt it would be that easy though.
Not that easy...but if the opponent were already dead...and somehow I were out of 9 level spells...I might be able to get away with it.
Does the opponent actually need to be dead in order for the Wish to stick?
No, but they don't get a save if dead. It would be a will save, so while reliable in the the long run, I might not survive his initiative.
Assuming you get to go first, I think the best way to shut down a caster would be to Wish a small stone with Silence cast on it into their stomach.
As mentioned by Adept_Woodwrigh that isn't going to work due to the 'rules of magic'.
Hit the vicinity of the caster with Persistent Mage's Disjunction. (2 slots). This will take down Prismatic Sphere, which at worst is 20 ft from caster center (Mage's Disjunction has 40 ft radius) and will continue effecting spells inside. (After all, it will hose anything inside if it manages to take down Antimagic Field, and Prismatic Sphere is no Antimagic Field) Use Quickened Wish to Wish him dead when your opponent has no defense against it.
This seems like the opening salvo of choice.