Farming Pink Tail

You can farm Pink Tail from Flan Princess, which is a rare encounter that can only be found in the 1st‑level room on B5.
Make a Quicksave before entering, then use the Siren inside to trigger battles with Flan Princess. Repeat the Sirens until you get the Pink Tail.
If you still cannot get the tail after 99 Sirens, teleport out and buy more Sirens OR reload the game from Quicksave and repeat the process (assuming you don't need EXP or GIL anymore).

Strategy:

• Cecil wears Crystal Mail to prevent Berserk.
• You also have 2 Ribbon in your inventory; put them on Rydia and Rosa to prevent Berserk so they can summon and cast healing spells.
• Both Kain and Edge will be in Berserk mode, so ignore them.
• Rydia summons Bahamut, and everyone else uses normal attacks.
• Flan Princess has 20000 HP, which requires 2 Bahamut and a normal attack to kill them.
• If you need to heal up, use the save room on B7.

Note: if you are level 99, you can simply auto‑attack these guys easily.

Note: If you want 5 Adamant Armor, you'll have to farm 5 Pink Tail, but you need at least 1 to complete the achievement.

Exchange For Adamant Armor

Head back to the surface and use the Hovercraft to access the Adamant Grotto. Inside, talk to the old man and choose Pink Tail. One Pink Tail for one Adamant Armor.

The best thing about Adamant Armor is that it has a whopping +100 Defense with extra stat boosts, immunity to all ailment statuses, and resistance to elemental magic.