oké, so I just completed the challenge, this is just for helping other players complete the challenge, the only thing you need to do is heal someone 100% of is health without him going down, so if someone is at 20% hp left you to heal him or her for max health so 100% you did 80% you need to heal him or her for another 20% to get it counted toward the challange.
the best method I found boosting this with 2 players is
game mode, hunt
Monster tier 1 everything on low so you get more time
As extra choose toxic clouds, so now your buddy is going to stand in it and just heal him, keep healing him!
If you have a third buddy who can play as the monster you should get it in 1 match.