If you go to almost the centre of the map and go to the super fight club (the hard one), then on round 2 of this is king of the hill and you'll have 6 minutes of non stop aliens spawning and running towards you so simply keep killing them with your freeze power. Freeze them and then shatter them and it will count towards the 100 you need. When you have the 100 done the achievement will then unlock!
The achievement for me personally did glitch a little as I met the requirements but it didn't unlock for a while afterwards so don't panic if it hasn't unlocked when you've got 100 kills!