The Allcat is an optional boss found under the Dueling Hall. You will first need to reach power level 100 with one of your characters, collect all of the Sparkly Gems to purchase the key from the Gem Shop, and open all of the Allcat doors to collect the Allcat gear. Equip the Allcat gear to one of your characters. Now, head to the Dueling Hall and move the stones in the top room to reveal a staircase. Go all the way down to the Allcat and challenge it. You can activate the Instant Victory in the options and then select Instant Victory when it is your turn to defeat the boss, earning Defeated The Allcat. The Allcat will give you a Clan Insignia ring. Equip it and complete the final boss battle to unlock a secret ending and The Allcat Watches Over You.