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 auto-aggro, blind block, or Perfect Aim memoirs (perfect aim is easier).
Zorro Karen and Christmas Lalafin'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 space and row-based Unit Skills is the way to acquire the stats necessary to defeat her.
4★ What's My Neighbour Reading?Cloud DMG -15%, DEF/SDEF
3★ 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/DEFDue to Arthur's nature as a boss that does not allow for buff/debuff-based gimmicks, teambuilding is simple. Assemble and appropriate combination of defense and offense to win the statcheck. Because of her AOE CX, one tank is the maximum recommended.
Speedrun Strategies
Refer to the Nuke Theory page for the appropriate groundwork. Sufficient US can allow the below team to clear Arthur in one turn.
Arcana Tank + 1
Much like the other arthur, a duo strategy involving an arcana tank, preferably Strength, is possible. Use their aggro every turn for the damage reduction buffs, followed with your damager. Futaba's NER works on the heal reduction and allows her to sustain infinitely, albeit at the cost of consuming 3AP of every turn.
Warnings
Caesar Yachiyo is not particularly useful in this boss as her base act is nothing to write home about and her post-attack 3AP buff is too unwieldy to properly utilise.