Each level of Shadow Tower (the secret Dark Guide Post stage) right before the boss fight has a bridge that needs to be created in order to get to the boss portal. In order to get each block of the bridge to appear, a certain number of "Key" monsters need to be defeated in that stage. You will know which creature is the "Key" as soon as you kill the first one, as a small cut scene will lead over to the boss portal and the first block will appear. Simply kill enough of the same creature and you will have a clear path to the boss fight.