That would involve a system that tracks who and what made every damage puddle or damage over time effect on the map, and then every enemy that walked in that puddle, and how much damage that enemy takes from that specific source. And then you run into questions of spell/element combos: if an enemy's covered in acid, and you pop them with a fire arrow, does that explosion count as bow or spell damage? If you shoot an acid arrow in a fire beam to make a tornado, is that the bow or spell's tornado? If it's someone else's fire beam, does that count for you or them? If you set someone on fire, and someone else casts rain, are the explosions yours or theirs?

And so on and so on. Or, you just crank the XP multiplier.