May contain SPOILERS if you haven't finished game.

(Be sure to do this after Mimir's Well, and it would help if your levels are at least over 50)
(Also, I don't think you get anything special for this fight, but it does give you some valuable EXP)

Once you've obtained the Grappling Hook and gone back to the Ark of Destiny, I'm sure you've noticed that there is a higher level to the library. Use the Hook to get up there, and read the book (can't remember which one, sorry!) that tells your about the Ice Queen in Mimir's Well. Well, it's pretty obvious what to do now. Go to Mimir's Well, and get to the area at the very end where Werner typed in the password 'TOMORROW'.
Type in 'EMETH', and you'll automatically be thrown into a boss battle.

--- Lolithia
--- Resistant --- Ice
--- Weak --- Fire

A pretty easy battle. Start of by having Virginia using 'Mystic' + a 'Lucky Card'. This will give everyone double EXP. Make sure Gallows has the ability 'Weakness' and have him cast Fire Arcana. Also, if you use 'Attachment' in conjunction with the Fire element on your most (physically) powerful character (for me that was Clive) and have him/her attack, it will do some pretty heavy damage.
If you wanna cheat then just use Finest Arts a few times.
And there we go! Another optional boss battle out of the way. What fun.