Ok, let's figure this out.
Guzma's final team is all bugs, and Buzzwole's Bug/Fighting...
As much as I wanna say that Guzma will just end up catching this flexing mosquito for his team, he'll need a Beast Ball in order to do that. And I'm pretty sure that after a particular event in the games, Guzma probably wouldn't want to be associated with the Ultra Beasts.
As such, the best result will be that Guzma will fight this guy to the end. With how powerful Buzzwole is, Guzma might have a little trouble, but he might have a bit of an edge in the competition thanks to him having a Masquerain. The end result will most likely be Guzma winning by a margin.