Its alot of HP but there is a possitive side, you are allowed to leave the battle and restore your health/save and the Mark won't restore any health. Your team should be at Level 85+ to make the battle a more even fight. Use Expose for about 10 times to reduce its Defence and keep your party members spread out to avoid the entire party being wiped out in one blow. Cast Dispel regularly and use the standard Protectga, Shellga and Hastega ib everyone, a tank with Lure, Bravery and Regen; a damage dealer with Bravery and possibly Berserk; and, of course, a spellcaster to heal everyone and cure all status ailments. Equip Ninja Swords and try to land a few massive combos. A Yagyu Darkblade and GenjiGloves are a particularly potent partnership.

To make things easier, remember the following:
-Stick close to Yiazmat to prevent it from healing itself.
-If you leave the battlefield make sure no one has Regen on.
-Be mindful of the traps around the battlefield.
-Don't use Holy or Holy-imbued weapons.
-Equipping Bubble Belts helps free you from having to cast that essential throughout the battle.
-Save once in awhile as you can't do it in one go without the risk of losing all that you have done.