The notion that D&D is a combat game in which all else doesn't count is a trope, that many players do not agree with and some people here constantly work to correct.  

A DM who wants to award XP based on killing blows *has fallen into this trope.  You should tell the adM that's not the game you want.  

*But for your presence*, the minion would not have cast the fatal fireball.  The group is the author of the situation which kills the monster, so credit should be given.

I once threw a beholder at a level 1 party.  The beholder was a questgiver and the encounter was on rails, but if they had found an extraordinary way of interacting with that monster, I would have awarded XP. Obviously they couldn't have killed it.