Don't bother with Onion Knight. It's a garbage class and not really worth the trouble by the time you unlock its full potential.
There's really not a lot you can do wrong in this game when it comes to choosing class. Just experiment with what you want. You'll find that there are a lot of game breaking skills in this game (i.e. shirahadori) that makes the game a joke. Hell, the game itself gives you a certain 'TG' that can pretty much solo the game easily.
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https://imgur.com/4UolSwI