It has an awesome leveling up system. When you play a mission you level up depending on your kills. At the end of the mission you get to choose abilities using "experience points" that you gain by killing people. In the next mission you get to use those abilities and it's awesome.
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Sing my world into being, Ainur. The universe needs your power!
It is always who shoots first, but if you stay close to a wall or do a lot of moving or throw a granade then you can beat them. It is all about skill. And it is skill most of the time if you shoot first.