I believe the only way to K.O. your opponent in backstage brawls is with a strong grappling move, a strong strike, or an environmental (grapple) move. This is only applied when your opponent's health is low.
I use R truth. But i change his finishing move to Undertaker's hell gate. My tactic is smash paul twice on the tv.After that use my sig move on him. After that straight away lock him in hell gate. Using submission move seems more advantage.
Do this- 1. Take him to the weight lifter. 2. Hit him 2 times the press R. 3. Hit him 4 times with the chair, 1st time when he is downed with the weight lifter. 4. Pick him up and  him to the cupboard then press O. 5. Then pick him and hold  then release after some time. You should be done if you do it fast enough, and then he will be unlocked for u.:)