The rules of a Championship Scramble are two out of five men start the match, then after a certain amount of time, a new opponent enters the match. When the time limit runs out, the wrestler to have last scored a fall (pinfall, submission, KO) will be the new champion.
Unless you actually want to wait the 20 minutes for achievement, you shouldn't have much trouble with this if you set it to a 10 minute Championship Scramble. With the ten minute time limit, it'll give you 2 minutes to get an uncontested pinfall, and if you start with someone like Orton/Edge and enter first and fill the rest of the match with low stats people like Primo/JTG, you should be able to get an easy pinfall.
If you want, make a single character with horrible stats and make sure he enters second and you enter first.
You HAVE to get a fall and be the first and only one to do so. If no one does, the match will end in a draw and you won't get the achievement. Once you have your fall, just make sure to break up any falls and at the end of the time limit you'll get the achievement.