He's not going to let go until he can't detect breathing, heartbeat, or muscle twitch for long enough that he's sure she's not coming back up. He's gonna keep crushing until his fist goes right through her. His aim here is not unconsciousness, or capture, it's death.

He's pretty sure Avatars like her are too dangerous to let live, and he's got no qualms about killing. Even if this is just a shade, it's one less shade that can hurt people.