On Tier 5, mission 5 on Single Player Campaign, you must face The Vadrigar 1 on 1. To Defeat Him, you must take out the shield generators protecting him. To do this, run to 1 side of the chamber you are in, press R2 to snipe and use the BFG-10K to aim at the red balls. USING ANALOG MAKES THIS EASIER. When you have destroyed the shieldgenerators, the vadrigar will go for all powerups, i would try to stop him whilst exercising caution using strafing. Try to get quad damage. If the vadrigar STOPS dead, hammer him with a rail gun 3-5 times you'll defeat him easy. Otherwise use a chaingun with quad damage and haste.