There's going to be a point in the level where you're fighting through alleyways. Straight ahead in a busted building are two snipers, and there's a third sniper on a balcony to your right if you get to close. Buggers will fly in and you'll need to take them out. Once these enemies are killed you'll push up a bit, and in a tiny little alcove to your right you'll see a stealth elite with a sword. He's standing still until you cross his line of sight, so you should be able to hit him with the plasma grenade. I died multiple times at this checkpoint, and was able to consistently do it 5 or 6 times in a row.