【CAMPAIGN】Defeat 3 enemies with one shock grenade.
The shock grenade is one of the throwable secondary weapons that can be found and used throughout the game. Once you have one in your inventory, use or to equip it, and or and to toss it. The shock grenade is far weaker than the standard grenade, however it really shines when you toss it into water, able to take out multiple fish BOWs in one blast. However, you cannot be swimming when you go to throw the grenade (obviously a good idea), but you may need to jump into the water to draw the enemies toward you before jumping out and tossing in a grenade.
(On Normal): I was able to get this in Episode 6 while moving from the now completely flooded engine room, to the bulkhead controls as Jill. You'll wade through some flooded water and eventually get attacked by the piranha-type enemies. Run to the very end of the water flooded area and you'll run into a group of these guys, equip your shock grenades (you should have picked them up last Episode) and toss one into the water. Don't worry about shocking yourself, it doesn't hurt too much.
This can be done in the "Bilge Block" area of Episode 5-2. After pulling the first lever to deactivate the steam, you will have to go through a flooded area with some piranha-like creatures. After coming out of the water and going up the stairs, there is a wooden crate with a shock grenade inside. Destroy the crate and pick up the shock grenade. Wait for some of the piranhas to jump up the stairs and throw the shock grenade at them. There's also a checkpoint right before the flooded piranha area. So feel free to restart the checkpoint if the grenade doesn't kill three enemies at once.