Ursa Major is the boss of the first campaign. Ursa Major throw's punches with it's fists and then exposes it's head which is where you shoot it to kill it. It also shoots rockets but they do very little damage. The easiest strategy to use is to sit on the shield and block it's punches while your CPU partner or 2nd players sits on an upgraded gun and shoots the head.
A couple tips that make this easier: Use any combination of beam gems and power gems on your turret stations, I preferred the power+beam rapid fire combo because the enemy doesn't move that quick and the turret, while moving slow shoots very quickly. Don't use the metal gems for this specific boss, this can make the boss much more difficult than it needs to be.
I also preferred the beam gem on the shield so I could deflect the missiles back at the boss and because there are a lot of red asteroids in this level, the beam gem on your shield will bounce you away from them.