Cumulative 50 kills with the Black Widow.

The Black Widow will allow you to fire up to six controlled grenades which you can detonate by pressing .

While working towards each weapon achievement, always hold onto a secondary weapon and press to switch between the weapons you want to earn kills for. Doing this will allow you to concentrate on the achievements you need without running out of ammo.

Multiplayer
You can earn all weapon achievements online with a second player in either ranked or player matches.

Weapons Training
You can find all the weapons throughout the campaign with little difficulty, though some are harder to find than others. If you miss any of these weapon based achievements you can use the Weapons Training ground to unlock the above list.

Before you can do this though you will need to collect all the Data Cells to gain access to all the weapons.

All your weapon kill counts will cross over between playthroughs and the Weapons Training so if you need to earn a few extra kills this is the perfect way to earn them.