Call of Duty: WWII

You use focus by holding in , and it seems that any rifle gives you this ability.

The focus bar lasts 6 seconds if used all in one go and takes 15 seconds to recharge back to full again.

You don't have to be aiming at an enemy or get a kill to get progress toward this achievement, so you could just stand there and keep doing it until the achievement unlocks. You'd have to use focus entirely 30 times and it'd take 10.5 minutes to complete the achievement.

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: