It is easy. Just try to keep together all enemies you can before star killing them in order to gain a high multiplier. Then, when you reach the boss dont kill him, kill the mininos he will summon until you reach at least 30k points (medium) or 40k (Hard). Also, to unlock the gallery you need to beat the demo mode at least once. Then, he best way to obtain the 4k points you need for the 3 stars is using dual pistols.