Jetpack
PRKO
Minigun

let all of the waves drop on the energy recharge field if possible!
After every wave try to get as fast as possible to the recharge field to fill your energy bc the jetpack will consume your energybar quickly, depending how often and long you use it

first get the attention of the cops by killing civilians
wave 1-3 can be easily done with the starter pistol
ignore the shield guys just shoot them after everyone is dead, they will switch to pistol by then.
wave 4 and more will have robots
while they drop down just fly in the air and after they dropped down shoot 2 times with the PRKO to them and land on the other end of the orange building ( that building, which is in the first screenshot on the upper hand side )
the robot will jump up there just wait for them like here:
the robot should move to the path so you can just prefire and get it destroyed fast. kill the remaining enemies.
wave 5 and 6 will have 2 robots. Start as the same way as you did in the 4. wave, fly while they drop down and shoot 2-3 times with the PRKO to their position and get on the orange building to the very end. If you feel like you are getting overrun just fly to the pyramide thing and try to kill some of them from there

You only need to complete wave 6 so that wave 7 starts. You should get the achievement with the start of wave 7

with this methode I have completed it easily on normal difficulty