All you really have to do is jump over SA-X, run to the other side of the roo, charge your beam and fire away when SA-X comes running up to you. Then jump over it and repeat the process. Sometimes it likes to jump when you do and intercept your jumps, you may take some damage there, but all in all it shouldn't be too hard if you lure it into the running back and forth pattern. When it tranforms into the butt-ugly Megabeast 3000X (Don't know the real name), just shoot it like 3 times with Charged shots to the face, then when it becomes a Core-X just shoot it when it opens its eye.