Really don't put the LV.X in. It can work, but you need a completely different build to support the gengar LV.X. Gengar is best at sniping your opponents Claydols/Uxies/Other important powers early game, and then sweeping with Poltergeist. Using the Gengar LV.X will destroy your speed. So build a deck around Gengar LV.X, or play a 4-2-4 line of the regular Gengar.

Credit for that Gliscor goes to Gliscor

The level X can help with sniping by finishing of those claydol and uxie that your opponent gaurded in time.
That being said there are better cards for this (realicanth, Hauter PA).

A really good addition I saw with this is Miasma Valley PL. The twenty is good for just if you are against a none grass or psychic deck for the spread power of compound pain.

The new Gengar w/out the power is NOT that good. Prerelease yes, but don't ever actually use it. Here is what I do:

3-2-3-1 Gengar
2-1-2 Ampharos SW
2 Crobat G
2-2 Claydol

If they play a supporter, then all you need to do is beat their face in w/ compound pain. And EVERY deck plays supporters.

OR:
4-2-3-1 Gengar (SF-SF-SF/SF/AoA(curse)-AoA)
3-1 Gallade
2 Crobat G
2-2 Claydol
4 poketurn
4 SSU

.