Magdor and Torsen have powerup abilities which lets them glitch-heal like that in raids. Ch17 has multiple cases of that.
Firehawk expends all hp into the fire beam and then turns into an egg. If you aren't able to hit it multiple times to kill it, it will revive to 50% hp. I recommend you or your guildmates build a team designed to kill that Firehawk in egg form. If you do, you will remove it from the battle.
Then you'll have to kill the other enemies so that all freeze/stun/slow/silence/control ailments are aimed at Magdor to prevent him from powering up, or, you'll have to out-dps the healing.