Crackdown 2

Thrust into the air and use a flocket launcher (like a rocket launcher but with funny rockens that like to move when shot out) and once you get high enough start spraying Flockets/rockets everywhere but this is only best done at night when freaks appear.

Best place to go is at -Unity Heights- because in the center (Middle part of that zone) there are massive group's of freaks that spawn.

Sorry if i make any spelling errors or left anything out that can be added. Just leave a comment if you would like me to add something new to the guide :) go agent

Quick Guide I created on how best to this and it is in fact a lot easier than you could think, Check out the video and it should help :-)

All you need for this is your thrusters and a good rocket launcher at night time.
The Shai Gen/Unity Hights island is best, since the most infected tend to be there.

Simply thrust up, and fire like hell! You should have plenty of time in your thrusters to kill 50. If not, start your thrusting from a slightly higher building/area for a little bit more time in the air.

what you need to do here is get the Turret Flack and finnd a massive group of freaks. I boosted in the air shot 2 rockets and got the cheevo, pretty simple.

P.S can i also add that gaining this achievement unlocks the green agent suit (Avatar award)

This is much easier than it sounds!
Grab a flocket launcher and make sure thrusters are equipped.
Go to the Unity Heights Square area when it is dark and climb one of the tall buildings around the square, the bigger the better.
Jump off and start firing at the freaks on the ground, tapping A to maintain height.
If you run out of fuel, let yourself drop a bit, this will recharge your thrusters and give you a little bit more time.
After 6 or 7 hits, the achievement will unlock and you will be rewarded with a green agent suit!

The description is deceptive - you do not need to use your thrusters (rocket boots) as a weapon to kill 50 enemies in one flight.

You only need to kill 50 enemies in one thrusted flight.

A good way to do this is to get your agility up to the maximum; then your suit's thrusters are unlocked and you can use them at any time, even if you have another explosive weapon selected. Choose the Quacker or Satchel Charge as your explosive weapon, then head to a street that's swarming with freaks (like the road along the south side of Unity Heights, the northeastern section of the city).

Throw down a series of explosives with (RB), spreading them out a fair distance apart, but in spots where a lot of freaks are. Boost into the air (tap (A) and then hold (A)), then detonate all your explosives by holding (RB). If you do this while you're thrusting, and your explosions kill 50 freaks (which is quite likely), you'll get the achievement.

One nice thing about this method is that you don't need to aim a rocket launcher - helpful if your controller is worn out and doesn't aim well, like mine!