I'm needing 4 more hard tokens but I don't know who to battle for a chance to get them. Can anyone help me with a few suggestions?

Also I'm curious what all those crystals do. I have more easy and medium tokens then I thought I did and if those crystals are useful I'd love to know.

Thanks.

Battle the golem that is out now. Even past 10 wins he drops them pretty regularly and it will bring you that much closer to the achievements. Other than that there's all of the normal challengers, Canaak - Pete and the secret challengers, Mama mush, Nependeathicus, night shade, and torroweep just to name a few. There are also cash shop challengers. There are so many of those though. You can try checking here: CLICK

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To answer your second question: The battle tokens are used to buy thing in the battle shop. Depending on which tokens you have, you can buy weapons, stat boosters, scrolls or weapon modifications.

The crystals are used to modify weapons. ">Here is a quick guide for using them ^^