I would apply general team strategies for most any character, but do it in a fashion that Mewtwo can best work with.
Here is one way to think about it. Suppose you have are on the ledge on one side being edgeguarded, but without being hit. Yet, your teammate is edgeguarding the other team player and successfully edgeguarding them. I believe its fine to simply stall on the ledge because your team is making a net profit. You occupy one enemy, but do not take damage or lose stocks for it. However, the other 1v1 situation is in your teams favor.
Make decisions which try to benefit your team as a whole. If your teammate is being edgeguarded you have to pull double duty somehow and interrupt the person beating up on your team member, but avoid being hit by the person you left vacate for coverage.