Time to break this down for ya.
Gengar: It becomes more effective as you gain trainers in the discard pile. I like this percentage of trainers, but you can't hold them if you want to make gengar effective. Also, call me old time, I would recommend using the Base Set 1 Gastly, it's going to be effective using destiny bond to keep you from losing it and making a Gengar ineffective.
Metagross: Gravitation is extremely effective. End of story. DO NOT PLACE IT WHEN YOU ARE IN A BIND! If you place this when you are struggling, your opponet will just take advantage of the lack of HP. Also, I would run more Stadiums, and less of the other trainers. Geo Impact will work well when there are stadiums in play, due to the fact that a lot of people will run mono color decks.
Claydol: Cosmic Power is another effective pokepower. The ability to draw cards can help when your in a bind. This could combine well with metagross. If you can use Cosmic Power, you may be able to get stadiums (which I once again stress you should run more.) Then Metagross becomes more effective. Keep this on the bench if Metagross is in play.
Mewtwo: The heart of the operation. Psybarrier is going to be really usefull in the next few sets due to the increase of level ups and the decrease of evolutions. Giga Burn will kill you though. You should run about 25-30 energy, so I would recommend dropping some trainers/pokemon and increasing your energy usage. Also, I can't make recommendations on what to do if you don't state which Mewtwo you are using, though I once again recommend you refer to an old card in the Black Star Promo 3 card of Mewtwo. Not only is 70 HP a decent spot for even cards today, but the fact that you can use it's attacks while you have Mewtwo LvX out can allow you to Giga Burn and then Energy Absorption 2 of the Energy cards back onto Mewtwo (the opponets will most likely not be attacking allowing you to execute this attack due to the effects of Psybarrier.)
Uxie: Drop it and put in an energy.
Spiritomb: If you get this card out your opponet will lose. Just keep it out and it your opponet doesn't get any trainers, and you get to evolve your pokemon. I didn't mention it before, but you should add some Potions. (I'm not going to go through the trainers.)
Azelf: I'm kinda back and forth on this one. You need the energy, but the ability to look at prize cards will help you with this deck, because, well obviously you'll know whats sitting there. The attack Lock Up isn't that great, so I would use this pokemon only for a benched pokemon and a wall.
Anyways, that's my analasis of this deck, if you want I'll go throught the trainers and tell you what to drop and what to keep. I still recommend you add some energy.
