You can repeat it until it die. But at theend, you will just get the maximum choco-heart it give (=max HP/1000). His life don't refill.
So, if you die once on a boss, you'll need 40 choco to get the max heart it give. If you die 10 times on it, you'll need 200 choco for his hearts.

Also, for my friends, I changed my helper squad. Put Kerria&Brice Camelia for damage/healing + Cherry sage for atk boost and debuf + Cyrtanthus for atk and skill boost+Chrismas Rose for more atk boost.