The notion that D&D (or a D&D based game like Pathfinder) is a combat game in which all else doesn't count is a trope, that many players do not agree wit hwith and that 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.