Although you will likely unlock this through natural gameplay, you can actually snag this achievement as early as the second mission,
Operation Cobra. During this mission you will move to a vantage point overlooking some German artillery positions. Pick up the sniper rifle here and use your
focus ability (hold breath, slow time) to take out the enemies.
Once you've cleared the artillery crews, your tanks will push through a berm -
DON'T FOLLOW YET! You can look through the openings the tanks created to snipe enemies using your focus ability. As long as you don't advance over the berm there will be unlimited enemy respawns giving you unlimited opportunities to use focus. That's it! Once you've used focus for
3 minutes (each use is ~6 seconds) the achievement will pop. Note: you can also repeatedly ask Pierson to "spot" enemies which will count towards a different achievement: