You need 25k, 50k, 75k and 100k in classic mode.
While going through classic you should get this achievement and level 5. If not just keep playing it
Just watch the bar at the top.
It's your health and slowmo meter
LT is slowmo. Only use it when needed
and it's better to tap.
Killing enemies gives health.
Be aggressive and kill them fast.
Hold RT to shoot.
100% Achievement Walkthrough: