If its 5e, he can hold his breath for 1min + a number of minutes qual to the constitution modifier, which should be at least +2. That 30 rounds, with a lv 2 heat metal that's 60d8 or a 270hp average, with a lv 9 heat metal that's 270d8 or 1215 average. Water breathing spell is also super easy to get though one could say the inside of the kraken isn't exactly water.
How long can your knight hold its breath and how much fire damage are you doing per round? I think you may run out of air first. Definitely an interesting idea if it’s already hurt.
save