The only way to beat Pixies in this game (so far for me) is to toss Potted Cactus' dropped by Happy Johnny's. It's best if you carry at least 4 or more if you want to beat that Pixie in a chest.
Just because it says 100% doesn't mean you'll level up. That just means you're close to leveling up. If you check your stat screen it'll say like 649/650. All you need to do is kill one more monster and boom. Level up.
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