The room where you meet them is the same room where you get an Elixer from a chest on the right side. I believe the room itself is on the left, but the chest is on the right side of the room. Shortly after you go down the set of stairs you can see Ragnarok from.

Alright, let's see. 1/64 chance...that's .015625. So a 1.5625% chance you'll encounter Pink Puffs. That's when you do encounter an enemy in that room, there's a 1.6% chance it'll be Pink Puffs. Then, there's a 1.5625% chance that it'll drop a Pink Tail. So altogether, that's... A 00.0244140625% chance that a random battle will be with Pink Puffs that'll drop a Pink Tail. Roughly, one in every four thousand random battles in that room. I'm sure there are more equations on the probability of encountering a random battle per step you take, but I won't get into that.

Let's say you fight three batches of Pink Puffs a day. That's still less than a five percent chance of winning the Pink Tail. So to have a decent probability, you'd have to fight for at least two or three weeks, every day.

So there you have it. Fight four thousand and ninety-six battles in that room, and you have a good chance of getting it. Enjoy.