Sega Vintage Collection: Monster World

You need to acquire the four Legendary items:

LEGENDARY SWORD is created by bringing the Old Axe (from defeating the Ice Bomber) and the Fire Urn (from defeating Tyrant Dragon) to the blacksmith in Begonia. You'll have to do this no matter what since obtaining the sword is key to getting to the Dark Castle.

The last three times are found in the Dark Castle itself. The LEGENDARY BOOTS are out in the open on the right side of the mine cart track. You'll see this as you climb the rope heading into the area. You'll just have to jump off to get it.

While you'll walking on the mine cart tracks, start working on getting the LEGENDARY ARMOR by heading to the bottom left of the mine cart area and entering a secret transformation room to make Shion small (press UP at the end). Once transformed, you'll have to walk to the right, fall into the pit, and continue right while jumping over the green gremlins until you can climb the rope again and ride the cart again.

Now ride the cart again, jumping off at the last minute to reach the otherwise too high platform. Climb the rope, then ride the rope in the next area to fall down the pit. You'll see the chest with the Legendary Armor to the left, through the short tunnel only Pygmy Shion can squeeze through. Note that when you head right to exit the area, the game will force you to transform back to normal once you enter the door at the far side of the room.

The LEGENDARY SHIELD is out in view in the next room with all the spitting urchin creatures. Finally a use for Quake magic! Equip the magic, cast once to clear the floor, then ride the first rope up and cast it again to clear out the two urchins guarding the chest with the shield.

This achievement is for collecting all the Legendary Equipment in Wonder Boy in Monster World. To unlock this, you need four pieces of Legendary Equipment:

Legend Sword
Legend Armor
Legend Shield
Legend Boots

You get the Legend Sword from natural story progression, but the last three have to be obtained during the Dark Castle map.

The boots are found in on the mine cart during that portion of the map.

The armor is found by dropping into a pit after the spikes

The shield is the trickiest. You'll have to climb a rope that raises itself up automatically and then kill two of those sponge monsters. It's tricky because if you get hit, you're dropped from the rope and have to reload that screen. I got it by using the Power magic which let me kill the monsters in one hit.

After obtaining the last piece of Legend equipment the achievement unlocks.