Just unlocked this and can confirm the Fire Strike can be used against enemies that you've just hit with Earthshatter. It makes it super easy because you only then need to ensure you time the Charge correctly.
Video below:
Text instructions:
1) I unlocked this on Watchpoint: Gibraltar in defender Escort mode. Ensure you're in Quick or Competitive play, of course. The payload is in a nice, contained area the fight there seems to take forever.
2) Charge up your Earthshatter special through normal play.
3) Use Earthshatter on an enemy and knock them prone on their back(s) -- this is the Y button on the Xbox One controller.
4) Use Fire Strike on the enemies on the ground; it's super easy to hit them, obviously -- this is the RB button on the Xbox One controller.
5) Use Charge as the enemy stands back up -- this is the LB button on the Xbox One controller.
I recorded and exported with XboxDVR.com, so the Achievement doesn't pop on-screen.
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