Collect Dark Crystals by killing various monsters. Find a Save Crystal and enter the item screen. Use a Dark Crystal to go to the Shadow Tower.
Easy Stat Increase:
When playing as Sharline in the area before the research lab, locate the Save Crystal under the cliff by Royal Commons. There is a plant creature that always gives you a Blue Fruit there. You can obtain maximum stats early by killing the plant/monster, then go into the shop. After entering the shop, all enemies in the area have respawned.