This is very easy but takes some time.
Explosives include rocket launchers, grenade launchers, and grenades.
If you are solely focusing on getting this achievement, I recommend the following method.
1. Equip Lobber, Firefly, and Grenades.
2. Use all of the above to kill gang members.
3. When you run out of ammo, reload at a supply point and start at #1. It might take a few hours, but you'll get there.
:) Good luck!