Cliffhangers can only occur on the following stages..
-Flow (Though it requires more work)
I'm going to assume you're doing this on Scramble.
If so, have your opponent hit you on the barricade twice.
Upon the second hit, you will begin to fall off.
Hold on to the building using the "X or Triangle" button.
If you block the attack you will earn the trophy.
This can be boosted locally with a friend/second controller.