Another thing to add, when you first enter the final boss area there is a staircase off to the left @ 11 o'clock. If you sit on this staircase you can use the railings as a buffer (depending on his location) because the primes missiles won't go through it. I sat up there for several minutes and let the prime "wail" on me and aside from extremely minimal splash damage every now and then I didn't get hurt by the prime at all unless he teleported to near the entrance. Also, I think the doors that open to initially allow you to enter the prime arena should close after the boss fight actually begins. You can sit outside the main doors and most of the enemies simply leave you alone. You only have to deal with a captain and his squad of melee grunts and the prime shooting at you. Makes the final boss fight extremely easy when you don't have to worry about being flanked or the prime getting too close and you can just back off down the stairs towards the remains of the spider tank if you get overwhelmed.