The following achievements (including this one) are easily done on the High Road after Pripp offers you the side mission for the area and enemies spawn there.
You’ll want to do this on a non-Iron Mutant playthrough so you can save and reload).
This requires an Big Khan’s Ground Pounder mutation, which you should have enough to buy as soon as he joins your crew, so purchase and equip it.This is much easier with extra movement so equip a +Movement hat on Big Khan, you should have several by now.
Approach the Shaman and the two enemies in stealth, save, when the Shaman begins patrolling, wait until he’s between the other two enemies and Ambush with
, if you’ve done it right the Big Khan should be able advance with a single action point a spot where he can hit all three with Ground Pounder (it’s a 5x5 effect centered on him), as soon as the skill lands the achievement will pop and you can reload. If you miss just reload and try again.