First go to the cave, then go to the place where you fight Babaki the demon with a trident. Make sure one of your fighters are level 14. Keep beating that and you can get to a high level.
To max out Yuskay easy, beat younger togoro and elder togoro and then use overdrive on the younger when you fight him again, Yuskay will get LOTS of extra stats!
Max out all characters to get access to the boat which takes you to shadow arena.
Beat the first round to unlock Chu, Then beat the last level of the first area for Rinku.