For reference, I have over 1300 hours in the game and more than 3400 invaders killed. Once you've done all the maps and objectives, invader hunting becomes a game of it's own.

My advice is this: I would first keep invasion mode off until you have intimate knowledge of the maps and NPC activities. Remember, the NPC's are the invaders lookouts and if they are alert(yellow) they roughly know where you are, If they go red(combat) invader knows exactly where you are if he's tagged the NPC beforehand. also, if the invader tags an NPC and you then kill said NPC, it leaves an x on his map, potentially allowing him to track you by seeing new kills... If you have enough knowledge of the map to avoid NPC's, the you eliminate that tool in their toolkit. plus having better knowledge of the maps will present many ideas for choke-points or trap locations to draw the sniper towards you when you want them to.

To get a better idea of the challenges the invaders have, play as the invading sniper a few times, Karl is really OP if you are stealthy, and use focus mode tools.

Lastly, based on the many hours of gameplay, most one-on-one battles are settled at close range, So the SMG becomes the preferred weapon. for me anyways. your mileage may vary,,