As I've always said, Xeno's are stronger individually, but playing as a team the Marines will always win. The combined firepower is remarkable.
Play as a survival match as a Xeno, and pay special attention to the way the marines are moving - the Xeno's only tip it in their favour after one of the players gets bored/overeager and moves away from the group. Once that has happened the game falls apart fairly quickly for the marine team unless they have a sentry/power-weapon to make up for the loss in firepower.
My particular favourite trick in survival is to hide out by the sentry gun and kill the selfish guy that tries to get it first by himself for all the bonus XP. There's always one.
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Sgt. Jake Tucker - 13th Regt. 1st Batt. Freebooters.