Shrimp are water-based mutants that are introduced in Chapter 14: Dark Waters. Don't bother attempting this achievement while your on the boat, as you aren't in control of its movement, and it will be hard to kill any shrimp. The boatman will also be shooting/throwing grenades, as well. Wait until you reach chapter 16: Sundown. Here in the swamp, you'll find plenty of shrimps swimming around. Shrimps will leave the water and attack if provoked, so provoke some, and toss a few grenades. You do not have to kill 5 at once, but only 5 total. The achievement will unlock upon your 5th kill.