Hmm, i gues in free for all games all you need is a steady aim, a good movement technique, ammo awareness and good reflexes...

In objective or team games, add all that to a good team effort. Don't get heroic and go to kill the enemies by yourself; try to assist your mates. For example, if you choose a submachine gun, try to go along with a sniper to protect him in close combat, if the map is somewhere similar to Stalingrad. And so on, and so forth...
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Angrif! Angrif!
Achtung! Scharfschutzer!
Zu decken! Angrif!...