Brock's booking makes it difficult for him to lose cleanly without a meaning build in my opinion. The only way it could feel authentic and satisfying is if they present someone taking advantage of some kind of weakness of Brock's, but he hasn't been booked to show any meaningful weakness so it's difficult.

I'd say that his part-timer status and the clear sense that wrestling is just his way of paying the bills is the only hint of a weakness he has right now. I think it's believable that Brock's character could be guilty of hubris and not doing his homework on an opponent.

If they also show that same opponent doing things like studying tapes of Lesnar, or specifically asking for warm-up matches with guys that prepare him for aspects of Lesnar's game (for instance a Last Man Standing match vs Sheamus to prepare for Brock's knockout power, a Submission match vs Jason Jordan to deal with Brock's amateur skills, etc.) while the commentators bash us over the head with how good his preparation is, then I think there's a number of guys that could believably beat Brock.