Jurassic: The Hunted

I found doing Level One twice through level select and meleeing all the enemies will net you this achievement. It was quicker for me than having to go halfway through the U-1226 mission to only grab 7 on my attempt there.
Make sure to finish the level, as reloading the checkpoint will reset your in-game statistics to what they were before you started the level.
It is a very short level and can be completed quickly, so if you need to run it through a third time it doesn't take long at all.
When you see the herd of dinosaurs, and the screen reloads into a new area where you are told to find the location of the ship, it is safe to exit out.

During the level U-1229 you'll have to find travel though some caves to get to a Sub. Right before you get to the Sub you'll get teleported into a smaller cave where a lot of little dinos. These guys take one hit melee to kill and don't cause much damage. You'll kill about 20 of these guys in this cave alone. Maybe a little less. If it doesn't unlock just melee a few red raptors in other levels after shooting them once.