Adrenaline

Hold 9 Energy at once in a single turn (you don't have to spend it). Stack energy sources: base 3, plus any energy-boosting boss relics (each adds +1 per turn), plus energy cards - Ironclad's Bloodletting (+2 for 3 HP), Silent's Adrenaline (+1 and draw), or Defect's Double Energy which doubles your current energy. A run with a couple of energy relics and one or two of those cards hits 9 on a set-up turn easily.