The secret passage is in the room with the 2 treasure chests between a long vertical staircase. Go down the staircase and to the left. You should see a peninsula to the left with no way of accessing it. By walking across the black area to the right of the peninsula you should be able to get to the Ragnarok from there.

Also you want to be casting Curaja with Rosa every round when you're facing Zeromus.