Hi everyone!
We came up with a killer combo to stop Entei's BAD Pokemon Power: Fossil Muk/Goop Gas Attack. First, let's look at Entei:
 
Entei    80HP
Pokemon Power: Bolt Whenever your opponent's Pokemon damages Entei, but not KO, flip a coin. If heads, shuffle Entei and all attached cards into your deck.
FFF: Protective Flame During your opponent's next turn, prevent all damage done to your benched Pokemon.
Retreat: C
Weak: W
Resist:
 
The goal of this deck is to prevent Entei's Pokemon Power from sending him back to the deck. Use Muk to completely stop it, and GGA to substitute. Also, Protective Flame will protect Muk from harm. Don't worry about putting energies on Muk unless you have to. Here's a deck idea:
 
Pokemon: 22
4 Movie Promo Entei (Main Pokemon, sweet attack)
4 Grimer (to evolve)
3 Muk (Toxic Gas)
2 Blaine's Vulpix (Power keeps you alive to evolve)
1 Blaine's Ninetails (Pokemon Power rox, and 2F for 50!!)
2 Eevee (Flareon)
2 Dark Flareon (playing with fire gives possible 50, at least 30)
3 Fossil Magmar (no explanation needed)
1 Neo Magmar (1 energy slower than 'Chan)
Trainers:13
3 Bill (card drawing)
2 Comp. Search (get Muk/Entei ASAP)
1 Lass (hand disruption)
2 Prof. Oak (card drawing)
2 Energy Retrievals (Blaine's Ninetails)
2 Goop Gas Attack (Muk substitute)
1 Challenge! (load bench with Entei)
Energy: 25
17 Fire
8 Grass
 
This is actually our deck, and it does just as well as any other good deck. Just watch out for:
-Haymaker- kills Grimer, can shuffle Entei into deck quickly
-Stall- this deck relies on card drawing (Oak, Bill), so don't get carried away and deck yourself.
-Murkrow- can kill bench regardless of Protective Flame
Thanks for reading! send deck fixes, comments, or suggestions to mariomaniacs@home.com
~Mario Maniacs