Bloodletting

Jorbs

Ironclad has access to some impressively breakable energy gain, with Bloodletting at Common being one example. The tricky thing about energy gain is working out how to have enough cards to play with the excess energy. Adding more energy gain to your deck should usually go hand-in-hand with adding more card draw, for that reason. Cards like True Grit or Burning Pact can let you eventually exhaust your non-energy/card draw cards from your deck in longer fights, at which point a deck of almost entirely energy or card draw will potentially end up playing cards indefinitely.

Baalorlord

One of the best ways to get more energy to play cards. The cost in health is generally offset by the added value of the additional cards you can throw down. One way to win a run is to add enough draw to your deck to cycle this card repeatedly and use the energy gain to kill bosses before you run out of health.