I got this achievement with the Explosive Shot mod for the combat shotgun, so I'll recommend that you use that gun for this achievement.
To use a weapon mod, just hold
to activate its function, and
to fire it.
On the third mission, "Foundry", you'll notice five little circles in the upper right corner of your HUD. These are your Combat Ratings. These fill up each time you kill a bunch of demons. When a circle fills up, you get weapon upgrade points, which can be used to upgrade your mods.
Finding certain collectibles can also grant you extra weapon points.
The 3 upgrades for the Explosive Shot:
- Speedy Recovery (costs 1 point): Reduces recharged time between explosive shots
- Bigger Boom (costs 3 points): Expanded explosion radius
- Instant Load (costs 5 points): Reduces loading time for modification
Purchase all three upgrades and you'll unlock the Mastery challenge, which requires you to kill 20 Imps (the little guys) at point-blank range using the Explosive Shot.
Do this 20 times to unlock your achievement, and you'll have a neat little addition called "Cluster Strike" that increases splash damage when you use the Explosive Shot.
Please state any issues before downvoting, and Happy Dooming!