Youve gotta heal a hunter for 100% of his health without him going down. Example, if Griffin is down to 25% health, and you heal him to full, then youve healed 75% of his max health. Later on he gets zapped by a Mammoth Bird and takes another 25% damage. Heal that up and youve got 25%+75%=100% of Griffin's health.
If you heal Griffin 99% of his max health and he goes down, you lose all that progress.
I found this challenge came pretty naturally just keeping everyone topped off. It comes with time, really.