WWE 12

Open story creator and add a match "BRAWL -> ONE ON THREE FREE BRAWL"

Set a superstar against three random wrestlers, press Y on them and set their health to 0% (all body parts red).

Save the story, play it and just hit once every contender to win the match and the achievement.

Create 1 default CAW and move his sliders all the way to 1 before saving, copy and paste twice so there is 3 defaults.

Enter a Free Brawl match on Easy and the Achievement is easily yours after winning.

All you need to do is set the game on Easy, select Play, then Specialty, then Free Brawl, then One on Three, then select 3 weak characters. If you have the DLC as I do, I selected Michael Cole & JR. Then I picked Santino from the regular roster. Then just choose a strong character like The Rock, Austin, Brock, Undertaker, etc.
The quickest way to knock them all out it to use the interactive enviroments to throw them into and then just use the A button to grapple them.
They will each drop to the ground for good per grapple on interactive enviroments.
Once you take out all 3 the match is over and the achievement will unlock.

Go onto a speciality match and press "Free Brawl", then go onto "One on Three".

Tip 1 : Use a custom superstar with full stats to beat the easiest characters in the roster e.g - Micheal Cole, JR and Zack Ryder (he's got low stats and that broski needs a good boot)

Tip 2 :BUT if you are struggling then create 3 more superstars, and give them all the lowest stats possible.

Then have your AI reversal sliders set to the lowest and change all the other sliders to the highest.

When you have your match always attack the strongest (UNLESS you have used Tip 2 because there will be no strongest) Beat up your opponents by repeating grapple moves until they have the "Y" button symbol above their heads, then get them near a finisher zone (a crate, wall, soda machine etc.) and press it. Once you have done this to all three and knocked them out, the match will end, earning you your achievement!