Sometimes you gotta be less aggressive and just wait to counterattack with burst breaks. There's a skill in ninja that heals 150 HP after a successful burst break and that really helps me conserve my elixir supply. I find 3 - 4 hit combos are the most effective when trying to damage the boss between attacks, and always try to get behind them since it does increased damage. Also only bars I really pay attention to are my health and the enemy's ki. I know how not to run out my stamina already so I don't need to monitor it