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 wit h 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 DM this is not the game that you want to play.  

### Agency
*But for your party's 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 quest giver 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.