Arthur's main Artifice is her Ending Act - this Zeus-like self and enemy cleanse single-handedly kicks common strategies out the window, including MaxHP% DoTs. This reduces the battle to a simple stat check, where you just have to conventionally beat the boss with damage before Arthur wipes you. In aid of this, Karen carries buffs on each of her moves, which allows her to stack up to 60% defenses and 40% attack, making her even more difficult to defeat.
Her secondary Artifice of casting heal reduction further cripples absorb-based strategies, while her blind mandates the use of either Zorro Karen or Perfect Aim memoirs (perfect aim is easier).
Zorro Karen's party block blind passive allows you to use more offensively-oriented memoirs. All kinds of beneficial effect cleanse and stat buff removal can be utilised to lower Arthur's defenses and damage. Due to the nature of this boss as a stat check, there are few counters to what she does. Application of high moon and row-based Unit Skills is the way to acquire the stats necessary to defeat her.
The MLB PVP shop memoir Searching for a Four-Leaf Clover can be bought and placed on your tank to buy just a little more time. Musketeer Mahiru could be placed on a purely defensive memoir, or alternatively Perfect Aim + DMG down. If you are not using Zorro, the rest of your attackers should ideally be on perfect aim and damage up.
4★ Searching for a Four-Leaf Clover
Space DMG -15%, DEF/SDEF3★ Martial and Dramatic Arts
DMG Up + Perfect Aim, HP/ACT3★ Stretching Help...?
Effective Ele DMG + Perfect Aim, ACT/SDEF3★ Present for the Detective
DMG Received Down + Perfect Aim, HP/DEF1) I actually use tanks? (varies based on team composition and unit skill)
Tanks: one plus Musketeer Mahiru, or two. Attackers: two to three. Support, zero to one. Every single one will be moon, and rank 9.5+. Capitalise on unit skill synergy whenever possible, e.g. Castaway Fumi should be accompanied by backrow attackers, Vampire Shiori to midrow attackers, Awakened Futaba with seisho moon attackers.
Sample 1: BKRuko, Dockira, Pnana, Muskhiru, [MoonTank]
Requires medium to high unit skills, which greatly affects success rate. Phantom Nana and BKRuko should both climax twice - once with Doctor Akira's buff, once without. Phantom Nana may not need to climax twice if your US levels are high enough.
2) The only "cheese" that works against Space Arthur
Just use Heracles Maya's barrier ACT every turn, followed by 2AP and 1AP moves. CX when needed. High US on Maya is required.
3) I wonder if you can call this a solo anymore...
By boosting Tsukuyomi with enough unit skills, she can sort of keep up with Arthur's damage despite the heal reduction. But getting through all the unit skill sticks in front of her took so long that Arthur doesn't have much HP left anyway.
4) Glory of the Arcanae
Hermit. Yes, alone. Or add one unit skill stick if you need to. High US makes it faster.
1) Midrow - Defensive (100%)
Have the frontmost girl be a midrow HP US at level 19 or above; Musketeer Mahiru in second place, followed by three midrow moon attackers (one may be backrow instead, with a corresponding penalty to reliability).
2) Midrow - Offensive (100% w/ Vampire Shiori, 50% w/ Other)
Similarly, have a high midrow ACT US girl occupy the frontmost position, followed by Musketeer Mahiru and three midrow moon attackers. This strategy is significantly less reliable if the midrow ACT US girl is not Vampire Shiori, while not being much faster unless there are lucky crits.
3) Midrow - Hyperoffensive (100% w/ Castaway, 85% w/ Other)
Two high midrow ACT US girls, Musketeer Mahiru, and two midrow moon attackers. The below formation is completely reliable even with underleveled castaway Kaoruko. Once the front two fall, use Castaway's 3AP plus another attacking move until both Ruko and the other attacker are charged, then use their CXes in succession. Castaway's 3AP allows Musketeer Mahiru to only take damage from the first of Arthur's three attacks that turn. This strategy is very fast, clocking it at around 5-6 turns.
4) Ascendant Michiru - My destiny lies with the other king! (75%)
Pushing mid-offensive stacking to its utmost limits, and leaving but one girl to do all the damage - a four to six turn clear. Heavily dependent on critical hits.