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.
We made an expansion and it is awesome. Really, you should check it out, especially if you're looking for some extra challenge.
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