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!!!