Give your warrior the excalibur
Make sure his weight is 21 or the game will freeze later after a double kill
Don't use mimes or anything that can counter attack

Go inside
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Run around in here
Sometimes it takes a while for the correct monsters to appear

Your group needs to have Safe and Regen
During auto battle, the warrior uses normal attack and the other 3 defend
Warrior is front row and the other 3 are back row
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Your game will lag as it gives you over 999,999 exp
Don't run the game longer than 24 hours
To end the fight, kill all 3 monsters at the same time, so you avoid the game freezing
I use a summon to kill them all
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Use the inn at your submarine when you need to recover MP between AFK sessions
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The game has a small chance to freeze when 2 of the monsters revive in 1 turn, this is why you can only kill 1 at a time, and must kill all 3 instantly when you are done.
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I counted that I was getting about 385,000 total exp per hour - 96,000 exp per character
The video above was around 24 hours of gameplay
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24 hours is my safe stopping point, because I assume the game will eventually freeze from too much exp