Patience is a virtue. Knowing when to move takes some time. I ve been playing Stalker for a week here and there and I can pretty much do 2 15+ kill streaks in fights between bases. Just stalking cover points in the open and picking people in cover when no one is looking.
The cerberus is my go-to for general combat but i generally prefer the crossbow for extreme troll factor, as it's perfect for operating deep within the enemy lines. Beyond that I think basically any sidearm gets the job done fairly well.