Metro Last Light

The Easiest place I found to do this is in the "sundown" level. There are grenades found scattered throughout the swamp. Although, I would highly recommend stocking up on grenades in Venice so you do not have to go wandering. Do not worry about trying to get this achievement done when you first encounter shrimp on the boat ride, I found it to be pretty much impossible. The 5 kills do NOT need to be all at once, you can kill 1 shrimp with 1 grenade every time until you get 5. They do seem to cluster together when they come out of the swamp, so it should be easy to get multiple kills at once.

This achievement is actually very simple. A bit of luck is required and patience, but you'll get it. Much of my method was from the comment above so thank you Dr Scruffleton.

Firstly, start up SUNDOWN and run over to the plane. Go inside it and pick up the gas can to activate the auto save and to trigger the boss which bashes the plane. After that's done head outside and basically walk about around the plane to find the shrimps. This plane seems to be the main spawning point for shrimps, so you'll get this achievement in no time around here. They often spawn when you walk away from the plane so don't be afraid to venture away from it but return shortly.

Keep an eye on the big pond just outside the planes main entrance, this is where you're likely to find packs of 3 or 4 like I was lucky enough to find. If you don't get it on your first try or run out of grenades, simply reload your checkpoint. Also if you're not having any luck finding any, reload your checkpoint and try the pond outside the plane again as that's where it's most likely you'll find huge packs of them. I believe your kills also count if you reload your checkpoint so don't worry too much about doing it on your first try.

Ok, I decided to post this because I saw that many of the comments were much more helpful then the actual solutions....

1) The slug looking shrimp (they are the ones that spit acid) are the ones that count towards this achievement, not the ones with the arms

2) Run towards the plane to get the gas cans that trigger the "boss" shrimp, after he/she goes in the water, look to your left, you will see a grassy area, that is where the "slug shrimp" mostly spawn

3) You have to get all five kills in a single life, you can't just kill 3 and restart the checkpoint and kill the other 2

Credit: upindat, if I didn't read his comment in another solution, I would still be stuck!!!