There is a potential problem with Xerath's power in his relation to Sivir. Sivir is his big target right now and from what he has shown in his fight with Nasus that meeting should go like this:
Xerath: Good evening, my dear, would you happen to be Sivir?
Sivir: why yes, that's me! What a happy coincidence to bump into someone looking for me.
Xerath: Quite. (Melts her with a beam of energy of roughly the 2000 degrees F)
And there's really nothing Sivir can do about that. But if that doesn't happen Xerath looks like an idiot because he'd have to be to fail when he has such power at his fingertips. And if it does succeed or requires Nasus and/or Renekton to keep setting him back then Sivir becomes a protagonist with absolutely no influence on her story. Which would just suck for Sivir fans.
Having widely divergent power can be done. But it's very difficult to navigate that disparity.
The other option is to just ignore it completely and tell your readers to not think about it. Like how Batman has faced Darkseid several times and not died. People do accept it, but usually the interactions do make Darkseid look much dumber and weaker than he is when facing Superman and Wonder Woman.