This Achievement is pretty challenging so I do recommend having characters at level 9000+ with Rank 30+ items at lvl 300.
Firstly to unlock the Netherworld Tournament you need to clear all the stages of "New Party" This includes defeating
-Petta (Complete Pirate Trial in under 200 moves and Pass Bill)
-Prinny Kurtis (Need to find Green Prinny in Item Town 5 times and speak to him, then pass bill to fight him)
There are 4 difficulties that can be played however it is significantly easier the harder the difficulty is as you will be over-powered. The difficulty you can participate in is determined by your characters strength. With a LVL 300 weapon you are pretty much guaranteed to be in the top 2 difficulties. The difficulties are:
I dont know the exact levels but where each character lands depends on all their stats added up excluding HP and SP
There are 4 rounds and each round contains 5 battles. There are several rules which differ from typical play:
1) You must select a Leader. If your leader dies you lose the tournament.
2) You must select 9 other members to join you. They cannot be switched out and there is no base panel.
3) Items cannot be used and Equipment Cannot be changed during the tournament.
4) HP/SP and Casualties all transfer from one battle to the next.
5) If you lose a battle you will have to start over from the beginning of the current round.
Your best bet is to do this at max level with Rank 30+ equipment maxed. To make things easier, utilize the Duplication glitch with the Puppy paw stick to duplicate LVL 300 items and innocents. Simply done by equiping a Battle suit with the item you want to duplicate and nothing else. Load up the 1st stage of the game and use the "Run Wild" Skill with the Battle Suit. Kill the Battle Suit with the Puppy Paw Stick and theres a 10% chance you will steal the item. If you steal it, complete the Stage, if not, quit and hit retry. Upon stealing it, you will have one in your inventory and on your Battle Suit. Heal up, rinse and repeat