Enter 30 Years of WrestleMania and scroll to the right. There you'll find The Streak mode. Choose Defeat The Streak and choose any Superstar to face the Undertaker. When facing the Undertaker you'll want to avoid pretty much anything he does. You can use weapons, so I suggest hitting him with chairs and the ring bell to earn signatures and finishers.
The best way to beat him is to wear down a certain part of his body and make him tap. If he's hurt badly enough then he'll use the resiliency ability to escape, that's when you'll know he's ready to tap out.
Tip: If Undertaker earns a Finisher, make sure he hits them on you outside. Also, avoid his Comeback move. Good luck. See the video below if you're still having trouble (credit to DANALL3N for the video):
This one is tough, but advice would be take it to the outside and use the ring bell to work him over til his head and body are red. Barricade breaks help a BUNCH too. Then drop him outside one final time once you have a finisher and get in the ring and wait for him to come in. Then finisher and enjoy your trophy.
Edit your wrestler to have the running strike running neckbreaker. You can spam this move a lot (Taker hasn't reversed it on me once yet).
Also edit your Wrestler of choice to have a submission finisher (you'll want a head submission if you're going to spam running neckbreakers, body if you're going to spam sledgehammer attacks).
Big tip: even if his body or head are in a red condition you won't be able to submit him first time around and being in the ring makes you very vulnerable to reversals. Try your submission finisher outside and he'll use up his one resiliency leaving you free to submit him on your next finisher.
The easiest way I found to do it was to edit Undertaker's hit points so it's all on his legs then change most of his moves that he normally does. Like change his punching to women's punching and stuff like that, I also changed his finishers to armdrag's. I did it with Shawn Michaels, it was actually pretty easy after this, I just took him to the outside and built up 2 finishers and beat him.
Or change his whole moveset to a woman's moveset. That's probably easier, and then edit the finishes so it's just the woman's slap and something simple like an armdrag or something. Still can be a bitch, but it makes it easier.