This guide is a quick one about getting mad exp to make the game trivial. Also good for getting level 99's for the achievements.
Free Exp Guide
For setup you need to have colosseum unlocked and the plan activated as well as the Fairy Fencer F collab DLC.
Sometime during the game (can't quite remember when) there is an event in Planeptune which has some yellow teddy bear fellas looking to destroy your squad.
Get a full set of the Stone processor unit on at least one of your squad.
Do polytan fight now, they only hit with electricity and stone processor gives you 100 resist to it.
Keep doing damage and then free win because you are invulnerable the entire time you are in HDD.
After this fight they are unlocked in colosseum and can be fought again for free exp. (they are an S contract I think)
Well done, you have now cheesed the game going from virgin mid game dying to bosses to now chad killing final boss with wooden sword solo (warning results may vary when attempting final boss with wooden sword solo, I accept no responsibility for your save file if you don't save and get wiped).
And there you have it, free exp.