This achievement can be picked up whilst working towards entering the Ancient Robot dungeon SW of the map. This picture here should help you with trying to access the area that the achievement can be obtained:
https://imgur.com/a/COwSqKfTo explain the numbered arrows in that picture:
1. Head north out of Mutown through the most NE exit. Cross the rock "bridge" here to enter The Gulp.
2. Navigate through the rock section and cross the water (using Turtle/Mermaid/etc. form) to a small island, and descend the stairs here.
3. Make your way through the short underground tunnel and exit to the north at the end of the passage. You'll exit into a circular area with about 20 or so baddies. Kill them all and the achievement is yours.