multi takes some getting used to, in single player, you can point and shoot to defeat enemies. In multi it really helps to learn to strafe using the A + D keys. If you stop moving and then point and then fire at the enemy, they will probably kill you first. It is much harder to hit a moving target.
other technics: using shorter bursts with the smg to greatly increase accuracy.
Sometimes i and others try ducking, jumping, strafing, circling all at once - ends up looking like a dance
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