Jack has "seen" Melodies of Life before, through Gwen Cooper's eyes, used by the kid that just called himself the Onion Knight, so it's with the voice of experience that he can answer, "Tsubomi'll be fine, if this thing doesn't kill her first!" It's not a self-sacrifice.

As incredibly stupid as he finds the ability, the most powerful attack he's got right now is a Dragon Breath on the Left Side, like Chiaki said. She hasn't steered him wrong yet.