Find a door that will most likely be knifed through (Nelson Bay is a good map to do this on).
Throw some C4 on the door, when they knife the door, the C4 blows them up.
Have fun.
Two more tips:
Put C4 on the walls around the door aswell, just to make sure you kill anyone even near that door.
And on Rush mode, when defending one of the big tech type places, there is lots of doors and not many windows that you can climb through, perfect.
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