Flyswatter made easier
I was struggling with this at first but found it easier once I'd maxxed out the snipper rifle.
NB: I did this after completing the game as you won't get 'Living Legend' if you do it at the same time.
Select mission 3-4 on casual. Run round the barge picking up all the sniper rifles and you will be begin to upgrade. Fire a normal gun or let the floodlights see you and get yourself killed. Restart Checkout and repeat about 4 times. You get to keep your upgrades.
Apart from giving you extra ammo capacity and faster firing, one of the upgrades is a partial auto lock abilty.
When fully upgraded you're ready to give it a go.
Make sure you obviously take out all the floodlights (which is now much easier) and the guy on the first ship. If you miss him, turn around and he appear behind you. Use AR and take him out. I found it easier to do it that way.
When Elena shouts 'enemies incoming' aim at the two platforms in the distance, LT and fire. Hopefully you will get auto lock and take the first one out. Keep pressing LT and fire until you get one. Try AR if you can. If you've got the first one, the second one will travel across and drop down to the left. Keep aiming at target, LT and fire and repeat. I think I hit the platform which exploded and I got the achievement.
Hope this helps