Battle Square

2 different modes:

Wave Attack:
Fight through 10 waves of enemies. Each wave gets progressively harder. At the end of each wave, you can take your winnings and quit, or go all or nothing in the next wave, if you lose, you lose any winnings you already won. On wave 10 is a boss, Shiva, Ultima, Chimera. etc. Beat all 10 waves and get a Battle bonus. At the end of each wave could be a slot machine that either gives you a buff or debuff (like VII).

Can have the option to go solo, duo, 4 man, or 8 man it. Mobs will scale with amount of players.

Training:

In this mode, you can fight bosses solo and practice. Any boss you've unlocked, you can fight here. Solo battles only and bosses scale to be soloable. Learn the boss mechanics at your own pace. The boss will determine the rewards. Fighting An extreme primal offers more GP or whatever than the story mode version. Of course, the bosses do way less dmg and have way less health since you're alone in fighting.

Could use the Coliseum in UD as an arena since, it's not really used for anything else but a couple story missions.