Rengoku 2: The Stairway To H.E.A.V.E.N.

H.E.A.V.E.N.-A, -B and -C:
Beat Gryphus boss in level 8. Then, re-enter Level zero (Primordium) after
the intermission sequence and go to the door behind you.

Highest H.E.A.V.E.N. floor:
Finish all 99 H.E.A.V.E.N. stages.