The example wizard (>>65967982
) is immune to poison damage.>>65967911
You could not beat the example wizard in one direct confrontation, certainly not reliably.
I would buy the most charismatic, intelligent, and manipulative level 5's I could possibly find, and get them to search for allies and/or disposable tools that are dangerous enough in themselves to challenge the wizard - demons, dragons, angels, powerful kings, Gods, liches, and other magic users that have a grudge against the mark (no-one reaches level 20 without making a few very powerful enemies, or attracting envious and jealous eyes), and either using them to kill the wizard directly, or acquire some other form of assistance in the job (such as magical items, mook legions, more money for better PC's, etc.)
I would not be relying on the party to pose a threat to the wizard in themselves; I don't see how that could be successful. I would rather use the party as a tool for gathering stronger foes that actually *could* pose a threat to the wizard, in and of themselves.
...Which actually sounds like the plot of almost every BBEG that's trying to kill the player, but whatever. Sometimes the bad guy wins, sometimes the "protagonist" starts their journey watching their mentor die. All that matters is that the target ceases to be.