Easiest way to do this is to sim up until Wrestlemania by going to the calender and using
to get to March. Once there, start the show and sim every match (
) until you get to The Undertaker match.
If he is not taking on Roman Reigns, edit the card (
) and put Roman Reigns in the match (when I did this, The Undertaker was fighting Randy Orton).
If you get to the Main Event of Wrestlemania and there is no Undertaker match, just edit the card, like above, adding The Undertaker (and Roman Reigns if needed).
Once the match is set, make sure to choose Roman Reigns. Now, simply beat The Undertaker. Once you defeat him, the achievement will pop