δ Umbreon is pretty cool. Its BODY is essentially like Ampharos-ex's POWER- except for special energy cards only. Whenever your opponent attaches a Special Energy card, they put 1 damage counter on the monster. This would include cards like Blastoise-ex that attach multiple energies at once, resulting in 1 damage counter each time a special energy card is attached (the one POKéBODY per turn refers to if two δ Umbreon are present; only one of them can use its power, consistent with the ruling for Dark Ampharos's Darkest Impulse). This hurts LBS pretty badly; 2 damage counters for each Holon's Electrode/Magneton after the 1 energy-per-turn attachment.
δ Mightyena could also do pretty well, since it is the same type as δ Umbreon. You could make a swarm deck with it, since its attack is similar to Dark Ring. Couple those two cards with Dark Steelix and you could do pretty well.
Many players will try to steer you towards turn-2 type decks and popular archetypes. While there is certainly nothing wrong with that, I feel that overlooked cards and strategies could use more attention, since it gets tiresome to play against the same popular decks over and over again. For me, I tend to enjoy playing original/rogue decks more than pre-constructed/tested/archetype decks, since I prefer playing close matches on the edge of victory and defeat, rather than matches that shut out every single opponent.
That's how I feel. Ultimately, the decision is up for you to make. I hope you find this information helpful.