It's calculate with full xp. So it means if a kill give, let's say 600xp, you get -15% malus cause of dragon wish, so -90xp.
But if you kill an enemy in group you don't earn the whole 600xp, but an amount of xp shared between all hitters, something like 70xp. But everyone still gets the malus, so -90xp. That means 70-90 = -20xp.
Another epic fail in grindmasters NGD.
Anyway you could play with an xp booster to dont be affected by dragon malus. $$$$