Mutant Mudds Deluxe

First, you need to get Grannie by collecting all 2000 Golden Diamonds and 40 Water Sprites from the Real World.

Then, go to each stage and find its CGA-Land door and enter it. At its end, you will find a Grannie Medal. Collect all 20 present on the Real World Stages and the achievement should unlock.

Here's a tip: The stage door on the selection screen will be orange instead of blue where you have the medal.