Jurassic: The Hunted

Wait until the level Rise Above. At the start of the stage you'll see a bunch of Pterosaur flying about a bridge with some grenades to your right. Just chuck the grenades at the Pterosaurs in the air. Aim a little above them. I tried to hit them as they spawned on the left. If you miss just reload the checkpoint and try again.

Another easy place to get this is when you come out of the caves with the scorpions in them (after the Spinosaurus battle). You'll emerge in a small clearing with a tent in it and your character will remark that he has found some "Civil War equipment." Directly ahead of you are two Pterosaurs sitting on a rock outcropping not doing anything. Toss a grenade or two up near them and you should kill at least one of the leathery beasts.

If at first you don't kill one of the Pterosaurs, reload the checkpoint and try again.

I tried the other solutions and wasted way too much time trying to get this one.  

After you start the level "Rise Above", you come to some crates and a log you must cross.  Now, if you need the "who needs a bug zapper" achievement for killing 50 of the buggers on this level, then shoot every one of them that you can see, otherwise you can cross the log.  

Right after you cross the log you come to a rope bridge that you must cross.  Just as you start to cross, you will see the flying buggers start to fly towards you.  Just as they start flying towards you, you should start chucking frags a little above them and 'bleep'...'Death from below' unlocked.

Hope it helps.

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