Immunity is hard. The key to avoiding this is to not be hit, not be critically hit, and finally not fail your save. Those are three areas you can focus on to stop his one feat from affecting you.
Getting the Fortification ability on your Bracers of Armor can prevent him from criting. As a wizard, you already have Mirror Image, Displacement, Greater Invisibility, Blink, and similar spells to keep him from actually hitting you, even with that natural 20. You also have spells like Fly and that can keep you out of reach.
Assuming the paladin is also level 18, then it's a DC 28 Fort save to not be stunned, which is tough for a wizard, but the Great Fortitude Feat gives you a bonus, as does a Rat familiar, and the Improved Great Fortitude feat will allow you a reroll when you need it.
Investing in a Belt of Indomidable Constitution will get you more HP to help you survive the fight, and will bumb up your save as well. However, you can save a decent amount of gold by casting Bear's Endurance instead (if you can do so before the fight).
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You can cast Iron Body, which makes you immune to critical hits as well as lots of other nice stuff. If you have the arcane armor training and arcane armor mastery feats you reduce the arcane spell failure change while the spell is active to only 15%. You can also load up on spells without somnatic components and use the still spell feat/rods of still spell.
You can also use elemental body III or IV to become immune to critical hits.
Both of these will protect you from any critical hit affects such as stunning critical.
And just FYI, any melee dueler is going to take a gem of true seeing...so don't count on miss chances from mirror images/blur/displacement.
I wonder how many rounds you each get to buff...that would be an interesting duel!