No need to restart the game tho. You can just beat it normally and win all the impossible fights in NG+ so you don't have to grind.
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New Game plus. It's a mechanic many RPGs have where, after you beat the game, you can restart while keeping your levels and inventory. I didn't intend the mechanic originally but added it on the full release so people can have revenge on all the fights they lost. There's a message after the credits explaining how it works.