Jurassic: The Hunted

Best done on a fortification objective as you will have unlimited grenades for this. Just make certain that the grenade makes contact with the dinosaur so that it explodes on impact. The achievement unlockes when the 20th dino dies.

Best level to do this is "Vortex Zero".

Beat the game as usual and use grenades where appropriate. If you still don't have it (I had something like 14 or 15 grenade kills total) then go to this level.

At the very start you will come across three raptors eating a pachycephalosaurus this is literally at the start of the level. Do not fall down that cliff--its about 4-5 feet and you can't get back up...thing is the Deinonychus can't either. Get the raptors' attention and they will eventually charge towards you and group up for a grenade (also an easy way to get "Pack Hunting" achievement). Wait where you are and maybe shoot a few sparse shots into the right area and three more will eventually come.

You get 6 easy 'nade kills each time you playthrough.