-4000G are required before you attempt this sidequest. After completing the Wind Temple, speak with Dic in Raejack Village to upgrade the Flame Shield to the Bolt Shield. This costs 1000G. Bolt Shield in hand, head to Jim's Shack in the northwest mountains and speak to the mustachioed dude to buy one of each key type: Red, Blue and Green. These will cost you 1000G a piece.
-With a pocket full of keys, go to the northern swamp. To the west of the Dark Tower is a cracked wall that you can blow open with a bomb. Make your way to the end of the mini-dungeon to find an adventurer that will trade your Bolt Shield for a Hero Shield.