Wait, use Yendor late game? But not immediately? I would think use it immediately, explore everything, grabbing 3 boost mana, and begin to fireball the boss. Kill weaklings till you level up, continuosly fireballing the boss every level. When you hit level 9 attack the boss, fireball, and then Chaos Avatar. Then attack and fireball again, hopefully killing him. Use potions if necessary. The rest is straightforward. Whupaz, fire, etc.
I speak chaos.