3 Mew Prime
3-2-2 Vileplume UD
1 Jumpluff HS
1 Mismagius UD
2 Liligant EPO
1 Muk UD
1 Chandlure NDE
2 Terrakion NVI
1 Shaymin UL
2 Victini (flip) NVI
1 Cleffa HS
1 Shaymin Ex NDE
3-2-2 Vileplume UD
1 Jumpluff HS
1 Mismagius UD
2 Liligant EPO
1 Muk UD
1 Chandlure NDE
2 Terrakion NVI
1 Shaymin UL
2 Victini (flip) NVI
1 Cleffa HS
1 Shaymin Ex NDE
Pokemon = 23
3 N
3 Professor Oaks New Theory
3 Pokémon Collector
3 Pokémon Communication
4 Twins
4 Sages Training
3 Rare Candy
2 Seeker
2 Black Belt
3 Professor Oaks New Theory
3 Pokémon Collector
3 Pokémon Communication
4 Twins
4 Sages Training
3 Rare Candy
2 Seeker
2 Black Belt
T/S/S = 27
4 Prism Energy
2 Rainbow Energy
2 Rescue Energy
2 Psychic Energy
2 Fighting Energy
2 Rainbow Energy
2 Rescue Energy
2 Psychic Energy
2 Fighting Energy
Energy = 12
Strategy
Mewbox uses Mew Prime to see off a variety of pokemon, depending on the match-up, then copy their low energy cost attacks. Since mew has such a low HP, vileplume helps slow down your opponent. Victini lets you reflip for liligants attack which can inflict special conditions, muk brings up a pokemon, confuses and poisons it, shaymin EX does heavy damage once several prizes have been taken, chandelure lets you spread damage as well as attack the active, jumpluff deals more damage for each pokemon in play, mismagius hits for 30 for each trainer in your opponents hand and terrakion is a great lightning deck counter.
Pokemon Analysis
You have way too many different pokemon. You'll be seeing off for several turns to get the right pokemon in the lost zone. I'd focus on jumpluff, chandelure and muk. Terrakion is a great attacker against lightning decks, so they should stay, just be prepared to lose a lot of energy if you need to retreat. You'll need a 4th mew prime since the low hp is going to force you to lose several in a game. I'd keep the shaymin EX in as well as a late game attacker.
Energy Analysis
I'd run 4 prism, 4 rainbow, 1 grass, 2 fighting and 1 psychic. You have to be careful with your energy attachment in this deck, so the rescue energy is going to hurt you rather than help.
T/S/S Analysis
All of your pokemon are basics, so 4 collector will really help your deck. I'd drop a black belt for a flower shop to recover your mews. Your trainers and supporters are already pretty consistent, so I wouldn't worry about any other changes. You should have some additional room in the deck after reducing some of your pokemon lines, so I'd look at adding a couple of juniper or even increasing your energy count.
Overall
The only significant flaw that I can see is that you focus on too many different pokemon rather than focusing on just a few of them and ensuring that you have enough copies of them to be able to see one off, even if one is prized.