In the mushroom level, when you get the rocket pack, teleport to the beginning. If you focus, on the right corner of the platform, you can see a rope. Follow it letting you fall and when it ends, there will be a secret room and moon will be there. Talk with her and leave or finalize the level. Now go to the level 4 or the last level and moon will be there in the beginning, talk with her again and go to the right part of the map and explore it. In one part when you walk you will fall in an other secret room, there will be three goblins, Now teleport to the beginning and back there, moon will be there with the goblins. You can wait the goblins change the position or keeping shooting them close to the door and the process will be faster. I hope i had explained it well :)