From what I noticed about Eureka, even if you tag a monster and others help kill it, it can reduce exp you get - if not completely eradicate it entirely. At least, this is speaking from my own experience while I was leveling there and trying to get Challenge Log progress.
There's a level discrepancy threshold that, if passed, you get significantly less experience from a monster or FATE. I don't know if exp would be reduced if someone just purely heals you while you down the monster, though. That would be something to test.

I honestly wish they had a level sync for outside of FATEs, like they did on XI, where you could all sync to the lowest level player in the party. At least then, you could fight enemies 5-6 levels higher together, get Challenge Log progress, and still get a good bit of experience (plus chains) from the monster killing.