リトス (Lithos)
Lithos has 65,000 HP, 100 Defense, and 300 Magic Defense. She is weak to Fire and Pierce-type attacks, while being resistant to Light, Dark, and Slash attacks. Despite Lithos' low defense, she has absurd evasion (140) that would make dealing with her via melee attacks a chore. However, it's doable, and as a result, there are a few ways to go about this boss fight.
Of note is that she has some VERY annoying negative statuses at her disposal, so you should be prepared to be ready for Petrification and Charm, as well as an attack(?) debuff. Lithos, amusingly, can be poisoned.
Recommended team: Sakuya, Byakuren, Sanae, Marisa, Patchouli, Aya as commander.
Using this team lets you completely skip every single issue that this boss could possibly give you. It pretty much becomes a basic land-manipulation strategy, which is surprisingly effective and more than capable of taking down 65,000 HP. Sakuya will need 25 points in her Time Stop and Greatsword trees, Full Moon Revenge (フルムーンリベンジ), Private Square (プライベートスクウェア), atleast 12 bombs (I ran 15 just to be safe, this can be done by having 25 points in her Time Stop tree (she gets +2 at 20points in, but you'll be running 25 points anyways), 7 points in her Illusions tree (I ran 10 in this tree for the increased double attack chance) (also +1 Bomb), using her sixth-tier greatsword (also +1 bomb), her second character armor (+1 Bomb), and giving her the +5 Bomb accessory, for a grand total of 16 bombs), and as much MP as you can give her without compromising elsewhere.
Byakuren needs Elemental Weapon (エレメンタルウェポン) and Weapon Bless (ウエポンブレス). She'll be using Fire when she uses Elemental Weapon, and will be reinforcing attack buffs after the first Private Square.
Sanae needs Sword's Wind (剣の風) and Prayer's Wind (祈りの風). This is IT. It's up to you to decide what else your Sanae should bring.
Marisa needs Phoenix (火の鳥) and Extreme Heat (極限熱量). Like Sanae, it's up to you to decide what else to bring. Equip her with as much +Fire damage gear as possible, though. You'll want her spitting out as much damage as possible (unless your Patchouli has Sungrazer Comet; see below). 25 points in her Astronomy and Magic Mushroom trees.
Patchouli needs, if you have it, Sungrazer Comet (サングレーザーコメット, level 67), but if you find yourself without it, then Agni Radiance (アグニレイディアンス) paired with Fire Crest (ファイヤクレスト) will do just fine. Obviously, Hot Spot Vulcan (ホットスポットヴォルカーノ) and Active Elemental (アクティブエレメンタル) should be used to get the land-manipulation gauge to max Fire as soon as possible, but without compromising speed.
Aya will need 12 bombs. Therefore, 25 points in her Reporting tree, giving her her first character armor, and the +3 Bomb accessory will get you to 12 bombs.
Recommended formation is the one Orin gives you, as it boosts all fire parameters. If you do not have this formation, then any formation which raises physical attack will be fine.
On the first turn, you should use Aya's 3bomb commander move to set up a Private Square. Have Byakuren use a fire-property Elemental Weapon on Sakuya, Sanae uses Sword's Wind, and Marisa uses Phoenix while Patchouli uses either Hot Spot Vulcan or Active Elemental to raise the Fire land. Once the Private Square activates, proceed to mow Lithos down with Full Moon Revenges. Once the three turns inside Private Square are done, repeat the process, but this time have Byakuren use Weapon Bless on Sakuya to overwrite Sanae's attack buff, while Sanae uses Prayer's Wind to boost Marisa's and Patchouli's spell damage. Rinse and repeat until Lithos is dead, and hopefully she shouldn't even last long enough to get an attack off.