Come to think of it, what's the rough history of the TFFA? How old is it? Is it like a high school or a larger type of school which fuses all types of schools together (elementary, junior high and high)?
My thoughts for the solutions...
- Using the Holy Grail and making a wish to undo Mereo's wish: Seems a bit too simplistic for my taste. For the Holy Grail, I was under the assumption that a wish that has been made cannot be undone even if a counter wish was made using the same sources. For example, one guy from DBZ was able to wish himself immortality (Garlic Junior I believe). If it was possible for the Z Warriors to undo the wish by collecting the Dragon Balls and making a counter wish, why did they not do so?
- Awaken the sleeping Gods/Goddesses. A good few of them would be required to alter the entire universe, obviously: A more interesting path as a lot of effort would be needed to force the mentality of said Gods/Goddesses to truly desire to stabilize things since direct confrontations will probably be met with high amounts of disbelief and denial.
- Use the powers of the magical locations, such as the Hinata Sou, the World Tree, MolMol, Pararakelse island, etc.: How many locations are we talking about and is there sufficient enough power to stabilize the problem?
- Re-create the Human Instrumentality Project, perhaps? Um... what good will that do? Reality and the universe as we know it will still implode thanks to the fusion of all the universes together.