Boss Tricks: SA-X

Notes:

The key to perform this strat is purely timing. You must fire the missile immediately after firing the charge shot, and after firing the missile, you must immediately let go of R to start charging as early as possible.
For the monster phase, simply stand in front of it, fire a diagonal shot while it's standing, then fire a shot upward as it jumps. You should easily be able to finish this phase in two shots.
Screw attack makes you immune to the ice beam core-x, even its ice beam shots, so if you're near death, abuse screw attack.