Water is definitely the best class. It takes the spot of support so you don't have to bring any of the other crappy ones and is a nicely customizable one too. You can field basically any combo team you want and pick allies that counter whatever element your enemy is weak against.

If this game is like Pokemon, then playing Water ninja is like playing as Ash. You control your three pokemon battlers, heal and support them, and switch them out for better ones if the enemy needs a different element.