Jurassic The Hunted

I'm not sure how many actual Pterosaurs are in the level, but there are more than enough to get this achievement. At the beginning of the level before crawling across the log to get on the bridge just shoot all the Pterosaurs that circle around. You'll get about 25-30 kills here easily. Then when you're crossing the bridge go slow and check the towers on your left and right and the cliff side. You'll see nests with a few extra Pterosaurs in them. Also, you'll get dive bombed by them often. Just kill all that you see and you should get it about halfway across the bridge.