Before the rotation, players are most probably going to run 3x Gengar prime and 1x gengar lv. x since he can use the abilities of the prime, which, as a matter of fact will help a lot against sp decks, which are heavily based on lvl. x's. Mew prime could be used, although i wouldn't consider attaching an expert belt to mew. here are some stuff that could be teched in lostgar:
4x gastly
3x haunter
3x gengar prime (hurl and spread damage)
1x gengar lv. x (definite spread, and level down)
4x crobat g (obviously to be used with some poketurns)
4x mew prime (backup, and easy ko'er)
2x absol g
2x absol g lv. x
2x palkia g
2x palkia g lv. x