To get the Frost Empress Title in Sailor Piece, defeat the Ice Queen boss on Boss Island. This boss does not spawn automatically. You must use the Boss Summoner NPC and spend Frost Relics, Money, and Gems to summon it.
You can fight the boss in Normal, Medium, Hard, or Extreme difficulty. Higher difficulty gives a better drop chance, starting from 0.65% up to 1.2%. Your Luck stat also increases the drop rate, so using better race, clan, and passives helps.
If the drop is unlucky, the pity system guarantees the title after 155 kills.
The Frost Empress Title gives a strong 87.5% Damage boost, making it one of the best damage titles. It is also required to get the Ice Queen sword.
To equip it, go to the Titles NPC on Sailor Island. Titles cannot be traded, so you must farm it yourself.