Plenty of mechanics can’t be countered by a class on its own. Usurper wins with Alchemist, who should be more than happy to throw him a heal. If you’re worried about attacks that bypass DI, you can always wait until the NK is dead.
There’s no such thing as a class that has no weaknesses. Let the class have a weakness.
Unless, of course, you’d rather have the class be immune to literally everything? lol.