Use Gardevior PK with Cresselia Lv.X. You can use the Damage Counter you get from Gardy's power moved onto your opponent's pokemon. Plus you get an extra energy. That might be the best way to play Cresselia Lv.X.
~Cyber~
Garde places 2 damage counters, not 1. :/
On another random note, you can't tech in Bannette EX, unless your format has Legend Maker legal in it. Most modified formats don't happen to have that luxury.
Cresselia is... Interesting, but I wonder about how much it actually can do. Maybe the Basic is incredible and we've all overlooked it. But Cresselia LV.X on it's own suffers from being...
1) LV.X (thus 2 are playable at most, which... is good for recovering from Garde, but bad in that they are not Holon Mentor searchable and must be active to be leveled up)
2) 3 energy to do anything (without considering the normal Cresselia. Needing less energy to do stuff is good. 3 energy to do 40 damage is... really bad. It's a fun finisher, if you could actually finish anything with it. If this was 50 damage instead, this would be SO MUCH BETTER.)
3) The 2X weakness to psychic is REALLY dangerous. Nidoqueen MT always one shots this thing. Opposing Cresselia's with a strength charm one shot it. Crobat MT one shots it. Flygon EX makes this thing shiver in fear with it's attack AND body. Just scary considering how much promise those cards might hold, and how very hard of a match up you'd have against them.
Now... What could it be played with?
Well... Garde's a good idea when you have 2 out, and the new kirlia lets you use 2 supporters a turn (sort of). What about an easy to fit Fighting Tech into that deck? Reducing anyone to 50 HP left makes Cresselia pick up 2 prizes should the thing come out, and Cresselia hold a Strength Charm.
What about a pokemon that lets you control what your opponent has active? Bring up pokemon without energy to lay damage, then bring up a weakened target to KO with Cresselia? After all, Water and Psychic Pokemon both have ways of reducing their retreat cost (heck, Holon WP can be used by both, so long as a psychic energy is attached). Problem with this way, is that you're running two Basic LV.X pokemon, but it could work should a good sniper be found. Octillery maybe?
But man, that energy cost is brutal... Especially with the need to have tools to scratch opposition. :/