Getting your fighter to level 18 for your class change shouldn't be too much of an issue, as your speed and strength will go a long way at the early levels. Try to keep your weapon updated and around your level, and you could probably just use basic attacks to get the job done. However there are a few important skills to dump points into to make sure you get off to a good start.
To be honest, you can make it to your class change just spamming Suplex, which will be your bread and butter skill until later in the game. If you'd like other outlets for your excess skill points, you can try out the various close range attacks such as Hammer Kick or Low Kick, or Doppleganger for some protection. Nen Shot is a good idea if you'd like to add a long distance blast to your repertoire of skills.
Suplex is an up close grab and smash that deals large amounts of damage and is difficult to stop - this skill will make boss fights easy as you can just "warm them up" with a few melee attacks and then grab them.

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