U can go 4 times in the room per run
after you go 1st time in the room (after you hit him a few) just unequip your armor and let you dying, then u can release at the start in the dungeon.
Setos is still in the room and u can run back to him in the room now its the second entry.
when he go out the room make DMG between 2x and 4x then he go back in the room 3th entry now repeat the step and you got the 4th entry.
So by 8 runs you are done.

On top of what the comment above says, here's two tips for high ilvl characters:
1. Do the dungeon in HARD mode, to give Seto a bit more HP.
2. Unequip everything (armor, accessories, engravings) except your Weapon
3. Use low-dmg skills, possibly under-leveled.

This is to prevent oneshoting the boss right away, as you will skip the segments.