Arena Runs are found in the "Games" section of the main menu. The arena was changed a bit in the most recent update, so now it works slightly differently.
When you start your run, you are shown three cards to choose from at the Common (white) rarity. You can click on each card to see what their abilities are, compare stats, etc. You won't actually choose one until you use
to "Recruit" that card to your team. You'll repeat this three more times for a total of four cards; one of each of the following rarities -- Common (white), Rare (green), Ultra-Rare (blue), and Epic (purple). You would be wise to try to pick cards that compliment each other with their abilities. For instance, if you choose a card that creates yellow gems, but then you don't choose any cards that actually use yellow mana, then your creation card would be useless.You no longer get to recruit one of your Over-Powered weapons, in the Arena, which was a source of stability and a big help, before.
Next, you'll start battling other teams, playing the puzzle game as you usually would. After the battle you return to the Arena result-screen which shows your team and your stats. If you won, you'll light up a sword icon that tracks your wins. If you lose, you'll light up a skull icon. There are six
sword icons (there used to be eight
, so this is a little easier than before), and two skull icons. If you lose twice (and light up both skulls), the Arena Run is over and you collect some minor rewards. This means that you can lose once, and still continue your Arena Run. If you win six times (and light up all the swords), the Arena Run is over and you collect some better rewards.
If you manage to win six times without losing any battles, the Arena Run is over, you collect the best set of rewards, and you unlock this achievement! Congratulations.