An easy (and non time-consuming) way of getting it as follows:
As your superstar, pick The Great Khali, who is the only "Ultra Heavyweight" on the game. Now, he certainly is not the best of the bunch, but his weight class gives you an immense advantage, especially if you pick the right opponents, which is the following point.
So after picking Khali as your superstar and setting him as the #1 entrant, make sure that the 29 other names consist of low-rated superstars (to make life somewhat easier), and more importantly only "Heavyweights" or under. Because of "Ultra Heavyweight" being two classes above "Heavyweight" (and three above "Cruiserweight"), Khali is able to eliminate all superstars (if there are no "Super Heavyweights" included) with a simple punch, meaning that the whole ring-out button mash will not be relevant anymore.
Furthermore, by picking low-rated adversaries, the damage Khali endures is pretty minimal (as he's a freakin' machine), making long-term survival that much easier. The only thing with Legend difficulty is the amount of counters performed, which after a while does get somewhat annoying, as Khali isn't the most dexterous, to say the least.
Whenever a new person enters the ring, just Strong Irish Whip (holding RB + A) them to the nearest vertex, launching them over in the process. To eliminate, simply move the right stick in any direction. On Legend difficulty, you'll be able to pull this off successfully at least 80% of the time. Try to avoid having more than three people in the ring simultaneously, as the risk is always there that you get eliminated when there are more superstars in the ring.
Just to see whether this worked, I tried this three times in a row, and succeeded!