This is a story-related achievement and cannot be missed. It will unlock upon defeating Mariska, the Stage 3 boss of the game. Mariska is very simple to defeat as long as you keep dodging with to the left or right to dodge get attacks during all 3 phases. You'll primarily be using your Chainsaw Blaster here to take her down.
- Phase 1 - She will shoot balloon attacks at you, you'll want to dodge these balloons the whole fight to get the Third Eye achievement. Shoot at her with your Chainsaw Blaster then after she falls down make your way to her and complete the QTE to go into her next phase.
- Phase 2 & 3 - Each phase includes more and more Mariska clones, simply keep dodging and shooting until she's down.
Make your way over to her and perform the final QTE cut for the finish.