Once you enter Mysidia, head to the weapon Shop and buy an Ice Bow for Maria or whoever is your Bow person and an Ogrekiller for Gus or whoever uses an Axe if anyone. If anyone uses a Staff or Spear, get the Power Staff and Flame Lance, then at the armour shop, buy either Thief's Gloves or Titan's Gloves for your party and Ice Shields as well if you wish. Next, head to the magic shop, as this place has a TON of spells to buy, mainly ones from other towns, but some new ones as well. Definitely get a Holy Scroll and if you wish, Wall and Barrier, but you really won't need them. Your call, though. When you're ready, go ahead and start your training. When you feel you're ready and have done enough, go ahead and enter Mysidia and in the town, head to the south part and look for some stairs leading down.
Head down them and once here, head west to find a statue of a Goddess. Examine it and use the White Mask on it and then leave this area. Next, make sure you're all geared up and ready, because now we're going after the Black Mask. If you pull up your Map, look to the southeast of Mysidia into the middle of the Sea. You see the VERY small piece of land with the yellow box on it? That's where you need to go. So, go ahead and head there now and once there, enter the cave on this tiny island.