Overwatch 2

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.

Reinhardt's Earthshatter ultimate ability allows him pound the ground knocking over and stunning all the enemies in front of you.

Once you build your ultimate, push forward towards an enemy. Try not to get surrounded or you will likely get killed before attempting this. Once near an enemy, use your ultimate to knock them to the ground. As soon as that's done, use your Fire Strike on that enemy. After that use Charge to pin them as they get up.

All 3 of these must be landed on the same enemy. You should also not attempt this on weak enemies as they will likely die before the 3rd ability use.

If you'd like to learn more about Reinhardt and this achievement, you can view this video:

The achievement description fails to mention this, so it's hard to catch on and the guides listed are pretty good, they've helped me a ton, but I'm making this just to say that you have to kill the character that you're trying to do this achievement on with the final charge in order for it to pop, it takes an insane amount of luck to be able to make sure they survive long enough to last through your other attacks and die to the charge but it's possible. I felt the exact same that many other probably do about this achievement saying that it had to be bugged until i realized that I had to kill the enemy for it to pop.

TL;DR: You have to kill the enemy with your final charge for the achievement to pop.

As the others mentioned, this is the buggy character achievement. However, we found a fix to this, and you don't even need to kill really. You can get this one against D.va. Have Mercy heal D.va while you do this and it'll work! The three of us easily got this to pop. The first two guys didn't destroy the mech, but I did, which is why we used Mercy and get mine to pop.