Dark Void

99 red balloons achievement - Kill 10 enemies while they are in an anti-gravity bubble.

An anti-gravity bubble is a zone created by the Magnetar, a weapon you found in chaper 2-3 "Prison Escape", used the first time to open a manhole.
The technique I used to obtain the achievement it's pretty easy: find a level with a lot of enemies in a bunch (I took 2-4 "Breaking Camp" just because it was the next one... destroy some UFO with AA gun for the footsoldiers to arrive), shoot the Magnetar on the floor in the middle of them and then a bomb (normal one, not the Pokeball! :-D) in the same spot.
You'll have to make them explode while floating in the air (NOT falling or lying on the ground!) for the counter to go up. After 10 enemies you should have the achievement.
If you use the level 2-4, note that you can have bombs checking the crate in the middle of the tents area (gray arrow on the radar, same one you use to change weapons).

IMPORTANT: as Genericchicken8 pointed out (thanks!), make sure to do this before you fully upgrade the Magnetar, 'cause at the second stage is so powerful that instantly kills, so you can't get the enemies to float in the anti-gravity field. And because the weapons upgrades are tied to your profile (not to your save file), you can miss the achievement! :-/

I hadn't read fbsarts/Genericchicken8 solution for this achievement before I upgraded the Magnetar . if, like me, you missed this achivement you may still be able to get it.
I orginally played this on the 360, so i fired up this game on my xbone, but disconnected my internet connection before starting it! My Achievments showed as zero, as i hadn't logged on in a while. I continued to play offline, started on casual and played the game through up to 'breaking camp' and followed fbsarts solution to get the achievement. i played on a bit to get some other glitched achievements, hit a checkpoint, connected back online and signed into xbox live on the 360 vitual machine and I did get this achievement.