Ikenfell

You need a power level 100 on any character, go to the dueling hall find the 6 big stones with marks on them, press a on all of them, go down the stairs, you will find the allcat, but you cant fight them untill you find all five cat accessories hidden behind allcat doors, to unlock the door you need to find the gem trader, give them 10 sparkly gems and they will give you the allcat key, the locations of all 5 doors are, the roost top floor, Northern dorms next to meracchi's room, stoc & barrol the first room with the old man, cemetery the grounds keeper's cabin, the final location is the great hall, equip all of the accessories on one character and return to the allcat, once you beat the allcat you'll receive the clan insignia. Used for "the allcat watches over you" achievement

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.

A full achievements guide.

There is a section for the Sparkly gems that will give you at the gem shop the Allkey. The Allkey opens the 5 doors ( another section in the video ) and inside you find cat equipment. Beat the Allcat boss and take the insignia. ( access that zone from the duelling hall - you need a character with 100 power to push some stones. )

Others, like Swag21lol, have covered getting to the Allcat perfectly so this only covers how the fight with him goes rather than reiterate already said things.

My party for this was Maritte, Gilda and Petronella. Maritte and Gilda were geared for speed with Gilda having the Skull charm and Petronella having the full cat gear.

The Allcat is a 2 stage boss and he starts with 3 minions. Periodically through the fight he will summon 2 more random ones. The minions have 2 concerning attacks, one that can move a party member somewhere at the back where they can get ganged up on and another that causes stat debuffs (power and defence). The boss teleports so traps are not very useful against him but would be useful against his minions which have to run around. The boss and minions also place no traps of their own.

In stage 1, his only concerning move is a guaranteed poison aoe. However a regen effect can overpower the poison so make sure to bring along Petronella, Ima or a lot of status clearing items. His aoe attacks cannot hit your entire party unless they are grouped up.

In stage 2, he loses his poison move and gets two more concerning moves. The first is a field wide aoe which shuffles everyone's positions; so do not assume he can't reach you at the back. The second is a single target doom spell which will kill the character after 3 turns so make sure you heal that immediately. He is also flying now so spells like Maritte's Meteor do bonus damage.

My tactics were simple. Gilda could soften groups of enemies up with Teleport and Shocker. Maritte had enough aoe's to deal with the rest and single target spells when there weren't any. And Petronella kept everyone healed and enemies poisoned; her Lifedrain spell also heals her and immediately adjacent allies.