The Multiplayer was very team oriented and forced you to work together to defeat your opponents. The only mode we played was Team Deathmatch. The marines would all huddle around each other and pick off the Xenomorphs as they tried to separate the marines. One on one a marine has a very high chance of losing to a Xenomorph mainly due to its "pounce" ability. During the match one of the developers, it was fans vs. developers as the aliens, transformed into a "Crusher" Xenomorph. This is basically a huge armored bull that could wreck marines with a charge attack. When a crusher comes out the tides turn, because all of the marines need to split up to not get splattered. This leaves the regular Xenomorphs to pick off the separated marines one by one. To counter the crusher Xenomorph, the marines have the smart gun, which is a heavy machine gun with a tracking HUD. The HUD shows the whereabouts of incoming Xenomorphs that are lurking in the shadows.
Here's the interview with Brian, senior producer of Aliens: Colonial Marines.