ALIENS!

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As alien you do travel in large groups. to attack you use your tail, clowns, and second pair of teeth (tongue).   

whats cool about being alien is that you can climb on any solid surface!!! so you can kill a marine when your on the fricken roof!!! Also, the predators cant go invisible to aliens because aliens have to use close rang attacks... so it would be like cheating if Sega let that...

A second thought, aliens run very fast and jump very high... and you can blend in on just about anything!!!

A last thought, when an alien gets shot or sliest the blood is still synthetic acid... so it would be like on COD 4 how you can do a grenade drop when you die...