Go back to Tanline Temple and sneak back into Shantae's dungeon escape path. Follow the path until you come out to a bright room with a chest in the middle. Jump up from the chest's position to climb up to a room with pots on both sides. Break all pots, get all gems and go back down and exit through the door. Then, return and the pots will have respawned so you can reap gems as many times as you wish.