This thread is about Pachirisu's effectiveness, not the fact that CL is an absolute mockery of the logic of game design.
I don't believe Shaymin is a requirement here, and I am in the camp that Magnezone Prime IS the way to go for abusing Pachirisu. You have okay donk potential backed up by a fast, powerful offence that in my opinion not only rivals Gyarados, but kicks it to the curb.
Why I say okay donk potential is that Pachi's power can't be activated when you start it (see pixies, crobat G, etc) so you not only need something else to start, you need a way to get it out of active, and the energy for Pachi to get the donk. Sure, no-trainer donks are POSSIBLE here (Crobat G starts, 3 energy starts, etc), but they're not that consistent. Pachi donk is not anywhere NEAR as potentially viable as Sable Donk.
Pachi donk says "if I get a god draw I donk up to 50hp going first". Sable donk says "on most draws, I go first and donk 40hp, 50hp on a special dark".
Pachi is a donk threat and a source of energy drops in Magnezone, and a possible replacement for Regirock. The thing about Regirock, however, is that it cycles your deck MUCH faster with Magnezone's power, because you can unload any card (to discard) rather than just energy (to play), and you can do it every turn instead of once.
The big problem with an energy heavy deck - ANY energy heavy deck, be it fire (anything running ninetales/typhlosion engine), or rain dance, or this, is consistency. Without Roseanne's you really need extra energy to make those drops, which reduces pokemon consistency and takes away slots. If there's a way to pull energy out of the deck (to discard, to hand) OTHER than energy search, it may make things more viable. Heck, maybe we should make this the Energy Strategy thread.