This is cut-n-paste from a faq...
Fire Phoenix shells repeatedly at him, until his
shield turns green. If he teleports down at you strafe around and shoot him once he appears. Sometimes he will create Skedar (they appear to the left), and baby Skedar (they appear in the center). Once his shield turns green, you need to blow out the parts of the statue behind him. First, the part to the upper right, then to the upper left, then to the lower right, the lower left,
and finally, the remaining middle piece. Keep in mind that eventually he begins attacking you again, and you have to shoot him some more. Once you blast off the final part of the statue, it will fall down, and impale the leader in the head. (Objective 3 complete).