Face of the Mountain should trigger on any nearby minion killed
[just so there's no confusion, this is just the last upgrade in the Targon line]
Just like the Coin line, respecting the 4 charges of course.
Problem is that in the mid to late game, you don't get the chance to proc FotM...like...ever.
If you keep the execute but make it work on any minion death in the vicinity, i'd have less incentive to murder my teammates when i approach a wave with 2 cannon minions just for a Cait to instaclear it with Q.
since it gives health, MR, and a shield for all nearby allies.
and
get early game for the very reason you've stated.