Defeat 5 enemies at once with an incendiary grenade. (Campaign Mode)
You might notice that's it's pretty tough to find an incendiary grenade lying around. Luckily, you're guaranteed to find one in Black Side 2-1. Missions don't change the available items, so replay the mission with Gargoyles and break the right most item box. When you pick it up, don't use and wait for the mission to finish. Then, use it on thugs in groups of 5 or more while wandering around in free mode.