Question about Tristana's mission 3 schematic and AoE damage

Was playing with some friends last week and the player on Tristana had explosive shot as one of their cards and we had progressed enough that it was activated. So the question became, if we kill enough multiple minions with one command card and it gets us a team gear, is there a rule for determining which minion is the one that explodes. When it came up with chain lightning we just did each jump one at a time so we would know, but when something like blaze hitting 2 minions or the hilarious incident of detonating a 3 stack fuel tank in the middle of a large pack happens we just kinda picked one. Ok I don't think that fuel tank explosion had enough surviviors to chain to but the concept is still there.