The easiest way to do this is to start up a One-on-One match, with infinite finishers for both. Use a large character like Big Show, and set your opponent as Brock Lesnar. Now, the key is to wait for Lesnar to do his finisher (can't remember the name) where he lifts you up over his back and slams you down. When I was playing, it was the first move he did coming out of the corner. Wait for the timing of the RT symbol to pop up, and if hit properly, should do the reversal with an immediate achievement pop-up. Using Brock is easiest, because he has a tendency to go to finishers a lot; other characters will drag the match out. As always, if you're having trouble, watch during matches for how he moves before going into a finisher move, in order not to waste reversals. The 360 version is far harder to see the RT symbols, but with practice it should be easy in no time.