He will skip a turn, and on the other next turns, will ask a random party member which limb would they want to remove. The player can't deny his request, they must choose between 3 options -- it's best to be equipped with an item that prevents limb loss. If the player keep saying to remove the same limb over and over, he will ask you to stop cheating, and select a different one.
To easily beat him you need to equip the Fluted armor which will protect against all limb loss attacks, blind him with Glass shards or Black smog and then you need to keep attacking the head until he dies, having extra turns and health regen would be useful here but it's not really necessary.
Pocketcat: “I've given these past few days some thought and... I can't really put my finger on it. There's something awfully familiar about all of this, don't you think old sport? I've got this weird sense of déjà vu from the most random of things. Like please do stop me if you've heard this before - There is an ominous moon staring down on us, this term 'Termina' thrown around awfully lot... ...The accidental inclusion of Bremen army to all this, the three day time limit... And did you get to see those lovely owl cultist chaps yet? It's all just so--”
Player: “...Familiar!”
Pocketcat: “Right!! I'm so glad to have met you! There is some sssserious brain leakage going on between the two of us!”
Player: “I don't know what you're talking about.”
Pocketcat: “Oh...? You're not seeing what I'm seeing? Hm. Ain't that a pickle to suckle on?”
Player: “...”
Pocketcat: “It's all just so familiar, isn't it?”
Pocketcat: “That has lead me to think how myths and legends are born in the first place. Everyone knows the classics that originate from mere fairy tales told to children before their bedtime. I would be one to know them better than many, if you catch my drift my dear friend. Time took those tales and kept telling them again and again. Like a good broken telephone, the stories took new forms depending on who was telling them. The living pictures we see on the silver screen would barely be recognizable to the sweet little children folk who first heard the tales hundreds of years ago, told by their mothers. It's a wild ride for sure. What do you think will happen to modern tales crafted straight to the silver screen in this day and age? Are we going to take elements from those stories and keep honing them to perfection as well? Let the good old broken telephone keep the good bits while we selfishly craft new meat around them? Are we going to see this ill-tempered (rightly so), pale old moon threat mankind time and time again with his strict three day limit? Are we going to dress into these silly masks and act the parts meant for the people that came before us? What do you reckon, love? I genuinely don't know myself...”
You read his mind... “I did it! I dropped the 'L' bomb! What a rush! I'd tap my back if my tapping hand wasn't so deep in my pocket... Maybe you can do it in my stead, you little eavesdropper? Or you rather dig into my pocket while I tap myself? I jest. I jest. Don't worry, we both share the same gift of eavesdropping. Let us get back to our subject.”
Player: “The stories will keep on changing with times...”
Player: “We will stick to the originals.”
Player: “There will always be someone to rip off popular stories.”
Player: “The stories will only briefly live in this day and then vanish.”
Player: “I don't care.”
Pocketcat: “Hmm... It's food for thought, that's for sure.”