Shadow is a mini-boss that will appear about half-way into the final level once you get to Boss Case’ lair. She isn’t a boss per say because to defeat her all you do is hit the buttons near where she lands. The first switch is to your left as you look at her. The second is to the right behind where some lava is. And the third is to the left again. Once the three switches are hit she flies away and the achievement pops.