A great place to get the "Fish In A Barrel" achievement is Episode 1 in Claire's campaign, during the "Escape from the facility" objective. You will have to go down to a basement to retrieve a key. In the room where you find the key is also an explosive bottle. Use the key on the locked door in the basement, and you will come to a room with a lot of small bugs/spiders (you reach this room through story progression and it is unmissable). Immediately after entering, go to the middle of the room and throw the explosive bottle to easily kill more than five of the bugs -- but you must be quick enough or your teammate will kill them. You can restart the checkpoint if it did not work on the first try.