Eidolon

Eidolon comes in two sizes, when you first meet him in his lair he is normal size, that is just big. Once you have got him on his knees all hell breaks out, there is an earthquake and the central structure break apart revealling a large glowing Chaos Sphere; this rejuvenates Eidolon and he grows to enormous size. Do not bother shooting at him because he will be regenerated by the sphere. You need to concentrate all you fire on this to destroy it, then turn to polish him off.