If you suck at Space Harrier and have no intention of getting gud at it, this achievement isn't too hard to cheese. Spamming dodge over and over makes you effectively invincible, so any enemy formation or attack pattern giving you grief can be side-stepped in this manner as long as it won't give you motion sickness. Pool your magic, and then dump a fat missile load on the faces of the bosses. The angel enemies you fight on the rocket that attack once each verse are relatively weak and easy to dispatch with big wicked weaves hitting them simultaneously, so these guys should be milked for combo points by enraging them in my opinion.
The final boss, the Temperance, should be shot at with missiles while he charges his laser because he takes more damage during this time.
Finally, remember you only need to achieve a Platinum medal, not a PURE Platinum medal..