I don't know if I'd say that, really. Xavius never really fought a Wild God, to my knowledge - excepting Cenarius, whom he corrupted via the Nightmare but didn't actually fight outright. That's really part of the issue one has when doing these types of comparisons because "defeating" someone doesn't necessarily imply you are more powerful than that person in a 1:1 context. It's entirely possible that Cenarius could just stomp on and kill Xavius if they fought head to head, as it were; but they didn't - Xavius used his poisonous connection to the Dream to weaken and subvert Cenarius well ahead of time, and then enslaved the poor sot and made Cenarius temporarily fight on his behalf. If a weedy 100 lb. nerd managed to get the drop on a 300 lb. Olympic weightlifter you wouldn't say the nerd was "more powerful" than the weightlifter, they just managed to surprise and take them down via stealth.
I don't know about your hypothetical fights between Khadgar, Jaina, Velen, or Guldan vs. Xavius either. I don't really think Xavius is anywhere near as powerful as you think, so I think most of those characters could probably make a good showing against him. He'd win some matches, and he'd lose others. Xavius by virtue of his Nightmare connection has a special bonus vs. Druids, but he has powers of his own besides this. Personally I'd think he'd lose more often than he'd win, although sometimes he might prevail through guile or cunning use of his powers.