Scene 9: Lumenosynthetic Reactor II
Start the level on 'Easy' difficulty. In the first room, clear all the enemies on the ground floor. When you proceed up the stairs, exchange your shotgun for the Bloodcleaver. Finish off the remaining enemies and proceed to the next area.
Upon entering the reactor core, you will receive the only checkpoint in the level. After, return to the title screen and continue the level, but now change the difficultly to 'Hard'.
Once the level starts, run straight past Thanatos and immediately head for the reactor, up to your right. With the Bloodcleaver equipped, melee the reactor. Should take about 5-6 hits to destroy it and no more than 30 seconds to finish the level.
Piece of cake!