War Drum

The lives you bring into a boss fight aren't bought: they come from your power. Match the boss's → 3 lives; reach 75% of it → 2; below that → 1. This drum is the only way around that arithmetic: +1 life in your next duel, capped at 3. It turns a desperate one-life attempt into a playable two, or a decent two into a comfortable three. It's the middle rung of the three duel aids, and the only one that buys something as big as a whole life.