You have but two goals: kill and stay alive.

Manual:
AWSD = Move & Climb
E = Throw grenade
R = Throw proximity mine
Spacebar = Jump, Double-jump
Left CTRL = Crouch (better aim, can't move)
1-3 = Change weapon
Mouse-click = Fire (rocket follows mouse)

Collect crates to restore health and ammo. Destroy as many enemies as possible for as long as you can stay alive...