Rocket’s Entei ex really didn’t have much of a chance. It did have a strong sniping attack, but it required heavy Energy acceleration to really work. No matter when it was used, however, there was always a better alternative to do the same thing. That really left Rocket’s Entei ex without a chance, and having problems against multiple good decks just made it worse.
100 HP was not good at all for a Pokemon-ex, as it was affected by Desert Ruins and could be quite frail for something that gave up two Prizes. The Water Weakness could be a pain against Ludicolo, Vaporeon ex, or the occasional Suicune ex, but that Poke-Body could end up nullifying that so it wasn’t much of a problem. The retreat cost was relatively low at least at just 1, so getting it back to the Bench wasn’t too hard.
Rocket’s Entei ex had a really nice Poke-Body, Dark Condition. If Rocket’s Entei ex had any Darkness Energy attached to it, the Pokemon had no Weakness. This was really nice, as it prevented Water-types from getting 2 Prizes too easily. As Entei could be relatively frail for Pokemon-ex standards, any help was nice.
Energy Link did 10 damage for a Colorless Energy and let you search your discard pile for an Energy card and attach it to Rocket’s Entei ex. Interesting idea, but having other Pokemon get Energy to it was usually much better. Entei wasn’t the type of Pokemon to want to sit up front for long periods of time.
Volcanic Ash was really the only reason to consider Rocket’s Entei ex. For the cost of 2 Fire and 1 Colorless Energy, it did 60 damage to one of the opponent’s Pokemon. 2 Energy also had to be discarded, which balanced out the fact that it was 60 damage to anything, meaning sniping the Bench was completely possible. That was great, but there was that one nagging problem…
Other things did it far better. Blaziken decks in RS-On had Blaziken ex, which cost 1 more Energy to use but did 100 damage instead of 60. 100 was enough to hit a ton of important KOs while 60 wasn’t, which made it far superior. Blaziken ex also fit alongside normal Blaziken quite nicely. HL-On had Blastoise ex along with Holon’s Pokemon to accelerate, but Steelix ex did 100 there. It was just hard to fit Rocket’s Entei ex into a place where it wasn’t outclassed, and it wasn’t amazing to begin with. This was probably best left in the binder and not in a deck.