WWE 13

Here is what I found to be by far the easiest way and will only take you JUST A FEW MINUTES and Will not even require you to adjust the sliders!

Go to Options > Gameplay > Match Options > In-Game Options

Change the difficulty to Hard or Legend

Now go back to the Main Menu and go to: Play > One on One > Normal Match.

Pick a wrestler with a Standing Finisher Rock, Stone Cold etc. (not off top ropes etc.)

When you hit "A" to select your wrestler an option box comes up that says "Attire and Finishers" go to Finishers and change it to "Infinite" with the D-Pad.

Select any opponent and leave their Finishers at "None"

When you get to the match-up screen goto "Match Creator" go to Win Condition and go to "Finisher Match" and change it to on.

When the match starts simply walk to your opponent and hit "Y" ....MATCH OVER!

When the final screen comes up just hit "Rematch". You don't even need to leave the match! Your achieve will pop up on the final ring of the bell following your 20th victory. Should take you no longer than 10 minutes. Hit start to skip replays for added efficiency. Any questions please message me. If you vote negaitvely please explain why and give me a chance to adjust it.

I did this by having my difficulty on Legendary and turning sliders down. Used Mark Henry vs Brodus Clay with unlimited finishers for Henry, and none on Clay. At the start of the match, Irish whip clay into one of the corners(B + corner direction) make sure he is STANDING but slumped in the corner and not SITTING slumped in the corner. You will be prompted to press Y for an OMG move. Once you do the superplex on Clay the ring will collapse(you can get the Holy S**T achievement if you don't already have it) and the match will end giving you a win by KO.

Do a rematch, repeat, and 15-20 min later you will have the Invincible Man achievement.