I just retried and beat it. You should prep a transmutation seal just in case a monster spawns in the corner. Play a goblin. Save BURNDAYRAZ, LEMISI, and BYCEPS for the boss. DO NOT use LEMISI before the boss. Use LEMISI and BYCEPS to whittle away as much resistance as possible. Prep +10% badge instead of RBS.
Remember this is supposed to be REALLY hard. It's a gold challenge.
I speak chaos.