Jurassic: The Hunted

I found the easiest level for me was level 6 (The Lost Castaways). The level starts and a couple small red dino's appear then you make a couple of small drops into the open field where around 6-8 small red and green dinos will come after you. Like stated above hold LB button to trigger adrenaline and hit the heart and they will drop in one shot. You can actually get this achievement and the Dinomite! one by reloading the level because they are both accumulative.

I found myself going into adrenaline mode more often than not, but your play style may be different.

Wait until you get a shotgun and just hold the LB button to trigger adrenaline and aim for the heart. You'll down most dinos in one hit this way. Wait for your adrenaline to recharge and repeat. Should be able to get it less than halfway through the game.