You will no doubt kill a few aliens over the course of the game but will likely need to grind the remainder.
An easy place to grind is within a Hard difficulty Super Powered Fightclub activity, there is one in the centre of the map (Shown in the video) start the activity and complete the first wave.
Now you'll start the second wave which is King Of The Hill, ignore the hill and instead stand on an open flat area. Several menu with bats will continuously come for you and agro to you now which is perfect as these still count as aliens. Make sure you have the Telekinesis super power selected with
and then press
to pick them up and once again press
to fling them away, hopefully they will die upon landing/colliding with a wall as they are low level aliens.
You have plenty of time on the counter to get all 100 but if you happen to fail make sure you EXIT the activity to start over else your kills will not count for your challenges (Though the achievement tracker will keep progress)
Here's a video