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Bomb your way to a 4-star Explosives rating

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.