Gyarados EX
Gyarados EX is a potential monster. It's a mere 2-stage evolution, meaning it can hit the field as early as your second turn, and its high energy requirement can be mitigated via its synergy with Misty - Genetic Apex #220 [U] and Vaporeon - Mythical Island #072 [AR]. With a bit of luck, it's entirely possible to have a rampaging leviathan on the field before the opponent is ready to deal with 140 damage volleys.
Note that the energy-detaching effect is 100% random and could target any card on either side of the field, so it's definitely not a reliable effect and it can be detrimental. Just be ready to re-attach some energy, and everything should be just fine.
Gyarados EXGo to Deck
A relatively basic example of a Gyarados EX deck. Articuno EX is a fairly standard early-game wall Pokemon that can dismantle early teams with a bit of Misty luck, and Vaporeon is included to effortlessly move Water energy around and result in minimal loss. Gyarados EX is the obvious headliner here with the normal support cards. Giovanni is also included as it allows Gyarados EX to one-hit-KO Mewtwo EX, thus keeping it from becoming a run-away threat.