Again, this is for Hard Mode, but normal can easily apply this:
Youtube video guide for Daisoujou
Pretty much what the video says and nothing more. Burn through your magic with buffs, and if you played your cards right, he'll do too little damage to be a threat when it all runs out, even when he gives himself extra turns later on. Buffing Agility helps the Death Spells to miss, but so does the Luck stat.
Do NOT ingest Magatama for Death/Expel immunity. At this point, the Magatama that do that will void one, but make it easier for you to get hit by the other, essentially dooming you.