BOOM!
Ultra rare0.4%GrindOptional
20 enemies killed by blowing up the surroundings. Save on hand grenades and enjoy the fireworks.
Kill enemies by blowing up stuff around them; explosive barrels and such. This can be hard to do as explosions don't really do too much damage.
Take every opportunity you can through the game, especially where enemies are bunched up close together - and where you can damage them first so they're more likely to die from the blast.