I'd have to say maybe either Oppression or Last Battle, Charge is good for a 2nd of 3 phase. Last battle works better as a final phase theme, so if you have 2 phases, stick with Last Battle. If one phase, I'd have to say Oppression or Charge depending on the bosses' difficulty and the severity of the boss fight. If it's serious and or a hard fight, stick with Oppression, if not, go with Charge. But if the boss battle has multiple phases, for a first phase theme, I'd say Gravity, Charge, or Oppression, but Oppression is a tough pick there.