It's real simple, play the arena when it's open, don't ignore it.  When you're at a lower level, the opponet selection offered by the game is easy to blow through, allowing you rank up quickly thru the divisions, awarding more crystal as you go.   You get 8 matches per run, which reset one by one every few hours or so.  But the cool thing is that crystals are awarded I believe every 2-3 hours.  I've been doing the following which has worked very well for me.  After you play all your first initial challenges (8 but you may a free challange scroll giving you 8 more),  log back in every 2-4 hours, collect your reward from previous wins, and then there should be 2-3 more challenges ready to play.  Rinse and repeat during the 3-4 day event, and you will make more crystal in a few days than any other method in the game that I've found.
   I will tell you that now that I'm lvl-30 with 2mil might, the long streaks of easy wins are gone.  I'm now getting served up opponets with double my might, and 10 straight re-rolls many times will not get me a decent challenge.  I have to lose several and demote down the ranks in order to see opponets that are beatable.  BUT EVEN THEN, I can still make a decent amount of crystal.

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