This achievement will pop upon completing the game, which requires defeating Medulla in level 5-5.
The battle consists of eight parts and takes place in a circular arena:
Phase 1) A large dome appears in the middle that you must shoot while avoiding blue orbs and lasers.
Phase 2) A repeat of the final phase of Embryo, the boss from level 1-5.
Phase 3) The dome reappears, this time circled by rotating red and blue orbs.
Phase 4) A repeat of the final phase of Vespula, the boss from level 2-5.
Phase 5) The dome reappears, this time circled by purple and blue projectiles.
Phase 6) A repeat of the final phase of Justice, the boss from level 3-5.
Phase 7) Medulla appears in the middle and is flanked by various hazards that must be avoided.
Phase 8) You are teleported to the Aethersphere where you must quickly destroy several waves of geometric enemies.
Here is a video showing my S+ run with Cactus: