I didn't use cheats or lag to win. Promise that!

All you have to do is go closer to your enemy and see were they are facing if they are facing up then go left or right and then shoot as fast as you can and you can dmg your enemy's hp to 10-30hp.

The secret was in there all along and you need a fast reload and decent freeze gun all along. Why didn't I think of that -.-

Also don't go far from the box and if their lagging keep shooting bullets or if he totaly freezed use melee to kill him/her. (don't go to the edge of the box. No escape go down or up the edge immediently or stay away from it.

Finally I got 6-3 already.