Kill a Suicider with a grenade.
The suicider enemy is found a bit later in the game, as are grenades, but the method is fairly simple. Just keep your distance and toss the grenade at its feet/in its path depending on if it has reacted to your or not. This can also be accomplished with anything that explodes (shooting a red gas can, throwing a Deo-Bomb, etc.) despite those not technically being grenades.