Not So Majestic Now, Sire
I was trying to polish off His Highness Emperor Ing in a 21% game, when he got stuck in a wall.
You can damage him by shooting his mouth if you are prepared to get very close and aim manually. After doing enough damage to his core, it changed colour and he continued the fight normally. I'm assuming this is caused by a clipping glitch such that the game waits for him to reach the ground before stopping the attack, but since he gets stuck in a wall, he never touches the ground, so the attack never ends.