Xenon Valkyrie+ (HP)

Prime Reactor can be accessed by giving Cells to the red guy in between stages everytime you see him while playing as Eloen(the white haired girl).

By doing so, after you defeat the boss in Thule Wulz Grotto you will see him a final time next to a giant drill, speaking to him will prompt a question asking if you would like to go to Prime Reactor (say yes) and, upon defeating the final boss in this location: the achievement will unlock.

Like other achievements: you can use the uber cheat code to your advantage () to skip through this stage and then nuke the boss with your 999dmg weapon.