In the room with the leavers first enter 1532 this opens the secret door under the arch. Pull the leaver in the room. Go back to main room and pull 3421 followed by the secret room leaver again. This opens the tomb on the final boss fight. You can then wait it out in there between each barrel protected from the AOE and the skeletons.

You can also enter these codes before the safe room one. Each time you need to pull the secret room lever before entering a new code.

1532 Opens secret passage in the back of the room
5123 Spawns resources on the tables
4361 Spawns a Rat Ogre
3421 Opens up the safe supply rooms in the final event (locks the levers)