From what I gather from Lancet's walkthrough;
Enemies hit and killed by "allies" (including charmed, but excluding your creature) give less XP. So the Goblin in your example gives less XP.
Enemies XP depends on damage you did to them. So in your example if the Chieftain never gets hit (unlikely) killing him afterwards should give full XP. Otherwise it gives reduced XP due to enemy damage done to the Chieftain.
Hope that made sense...
