One of the areas where you need to be Mini'd - but a difference at that point of the game is that you now have the Dark Knight job. If you have trained one already, Souleater will do great damage. Just have a White Mage in your party too for healing, and you'll just breeze through that cave.
1)Register someone elses's (or multiple) communication ID under Mog Net.
2)Send message to another PC 7 times
3)Send and receive 4 messages from Topapa, on the fourth one you will receive a message to do a mini-quest.
4)Go to Ur (First town in the game)
5)Talk to Topapa
6)Go to the cave up north of the cave
7)At the drop hole, you will have an event and then will be proceeded to fight 3 bombs
8)After the battle, you will obtain a piece of crystal, which will unlock Onion Swordsman
