You need 25, 50, 75 and 100 kills in Arena mode
*You need to reach 100K in classic to unlock Arena mode*
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 it
Killing enemies gives health.
Be aggressive and kill them fast.
Hold RT to shoot.
100% Achievement Walkthrough: