Step 1: Know when the enemy is about to get portal
Step 2: Watch invader spawn
If you do these two simple things right and have good aim you win. And its always better for the enemy to farm motes than deny yours. Plus you waste 30 seconds of one guy's time. If anyone loses its the enemy because they didn't set you back. Its easier to come back off of a Primeval too because you can keep invading. A lot of the stuff I see people complain about on here with Gambit is pure noob stuff. These things aren't an issue when you actually know what you're doing.
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There is a difference between knowing what you're doing vs having the edge over your opponents because you have a weapon that automatically gets kills for you. No other weapon in the game is like this, not even Golden Gun which is capable of the exact same thing but still requires human accuracy.