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doesn't know REAL wrestling...
Yes, Punk is too small when compared, ridiculously so, but I am disregarding that and I said so in the OP, my concern is how does a fan-favourite Punk believably beat Brock? And how do you do it protecting Brock's image for further storyline purposes as well?
Sure, he was beating the likes of Del Rio, Ziggler and Jericho but you seem to forget that face Punk was an absolute winning machine. It was only when he turned heel that he started to lose matches regularly.
How does Punk believeably beat Lesnar? It is rather simple. By following the tried and tested David vs Goliath formula. By having the same kind of matches/performances that he had against John Cena, who found it almost impossible to defeat face Punk. By having a similar match as Cena had with Brock - take a beating, fight on, hit a home run on Brock's head with the ring bell and then blast him in the face with a weight-assisted GTS. 1-2-3.
People are making it far more intricate than it needs to be. Vicious killers have been beaten by much smaller men before but still kept their image - Vader in WCW, Hell, it has even happened with Lesnar already in his current run.
And anyway, what is the big need to defend Lesnar? The likelihood is that after his feud with Punk, he will disappear for a few months only to return around Elimination Chamber to set up a match with Rock or Undertaker for Wrestlemania. His name alone will sell.