Arthur

Preferred IV: +SPD or +ATK / -HP or -RES

• An Attack or Speed Asset increases Arthur's offensive capabilities; Speed in particular has nice defensive applications as it blocks several enemy follow-up attacks while generating new follow-up opportunities for Arthur himself.
• At zero merges, taking a Flaw in Resistance is a sufficient sacrifice as he has mostly little to do with magical tanking outside of blue dragonstone users. HP notably makes Wrath and Desperation easier to activate if that is in your list of priorities.

Weapon: Arthur's Axe (+Eff)

• Arthur's namesake Axe offers him two forms of additional in-combat stats when 1) he currently has a bonus on himself 2) his health is below full or he is suffering from a penalty. Both conditions are fairly easy to achieve, improving Arthur's otherwise modest statline quite well by +8 across all non-HP categories. 

Assist: Ardent Sacrifice / Reposition / Swap / Reciprocal Aid

• To activate Arthur's Axe safely, one may opt to use Ardent Sacrifice or Reciprocal Aid especially in cases where there are no safe initial matchups.
• Reposition is a neat generalist Assist skill due to its single-space drawback, enabling Arthur to rescue his teammates without putting himself in their place like his default Swap.

Special: Bonfire / Moonbow / Blue Flame

• Arthur's Defense is already pretty good at base, and its potential improvement after taking into account other skills (such as Fury and Arthur's Axe) only makes Bonfire (which scales off Defense) a more considerable choice than other Specials.
• For single-round burst damage, Moonbow is a cheap and efficient choice, whereas Blue Flame typically offers superior damage and can be inherited from the same Owain if also inheriting Wrath.

Passive A: Fury (3 or 4) / Brazen Atk/Def / Brazen Atk/Spd / Life and Death (3 or 4) / Swift Sparrow 

• Fury is one of Arthur's best generalist A slot skills bar little else. Not only does it do well to visibly improve Arthur's rather modest, balanced statline, but it also helps activate the refined part of Arthur's Axe as it deals a small amount of recoil damage after a successful round of combat.
• A Brazen Atk skill of choice offers larger in-combat boosts at the cost of making Arthur more setup-reliant.
• Life and Death increases Arthur's Attack and Speed by a more extreme amount compared to Fury. The defensive drops and lack of recoil damage make it a rather pressed consideration, however.
• Swift Sparrow 3 offers the largest initial Atk/Spd boost of all listed options and works exclusively on initiation. 

Passive B: Wrath / Desperation

• Wrath is easily acquirable from Astram via Heroic Grails and synergizes with many components of this build quite well. In particular, since Arthur's Axe encourages Arthur to be damaged anyway, being at or below 75% HP would be a rather typical scenario. In addition to providing a neat +10 additional damage to Arthur's Special activations, Wrath also allows for the passive charging of said Special out of battle.
• Desperation is a cheaper alternative that allows for unhindered, consecutive follow-up attacks at a lowered amount of heath. As this is reliant on Arthur doubling his enemy in the first place, emphasis must be placed on Speed investment if using this skill.

Passive C: Atk/Spd Oath / Any Joint skill / Any Wave skill / Flexible

• The primary effect of Arthur's Axe gives Arthur in-combat bonuses if a stat bonus is active on himself. Therefore, C skills which allow Arthur to provide stat bonuses to himself will aid in Arthur's success. This includes most Joint Hone/Fortify, Oath, and Wave-based skills you are able to afford.

Sacred Seal: Brazen Atk/Spd / Sturdy Blow / Atk/Spd 2 / Any Wave skill / Flexible

• Brazen Atk/Spd offers an astounding additional +7 Attack and Speed when activated and does so at nearly the same time as Wrath and Desperation, massively improving Arthur's offensive potential.
• Other offensive seals are up for consideration; Sturdy Blow and Atk/Spd in particular are more immediate alternatives compared to Brazens.
• If you are running an Even or Odd Wave in Arthur's C skill, running an opposite-parity Wave skill in the Sacred Seal slot is something to consider. By running this, Arthur can always have a bonus active on himself and thus benefit from his Axe's in-combat stats at all times, though note that these bonuses can always be provided by other teammates.

Preferred IV: +ATK or +RES or +DEF or +SPD / -HP or -SPD

• To avoid the stat penalties of a Flaw (as Arthur values all of his stats), it is recommended to merge at least once regardless of level of investment.
• The Asset depends on your needs; Attack tends to lend itself well to tank-oriented setups, whereas Defense and Resistance subtly improve Arthur's physical or magical bulk respectively.
• Even Speed is respectable at high levels of merges/investment as Arthur can reach a respectable final amount with other certain skills. Thus, he can avoid follow-up attacks and even make his own without the help of Quick Riposte.

Weapon: Arthur's Axe

• Arthur differentiates himself from similar enemy-phase Axe brethren with his namesake Axe, which essentially lessens (if not outright cancel) the impact of penalties such as Panic, Ploys, and Chills. Arthur can thus be expected to keep himself fresh during continued onslaughts.

Assist: Reposition

Special: Aether / Bonfire / Sol / Noontime / Ignis

• Builds using Breath skills or any teammate-offered substitution of said skill (such as Brave Lucina's Geirskögul or Infantry Breath) vastly prefers Aether—which provides solid burst damage and self-sustain in one slot—as a Special.
• Otherwise, shorter-charged Specials with singular effects such as Bonfire or Sol are preferred with builds and team compositions that don't offer cooldown acceleration.
• Ignis is a high-strength option which can be saved more often for dire matchups as opposed to Bonfire.

Passive A: Steady Breath / Warding Breath / Fierce Breath / Atk/Def Bond / Atk/Res Bond / Sturdy Stance / Mirror Stance

• A Breath skill of choice accelerates the activation rate of longer-winded Specials such as Aether and Ignis, giving these skills more practical timings. The choice between the three currently available ones depends on which of Arthur's stats you value more in enemy phase, as well as availability in your barracks.
• An Attack-based Stance or Bond helps Arthur keep up an offensive presence while taking enemy onslaughts continuously. Recommendations of note include Atk/Def Bond, Atk/Res Bond, and their dual stance equivalents as they also improve a chosen defensive stat.

Passive B: Guard / Quick Riposte / Lull Atk/Def / Dull Close / Wrath

• The delay of the enemy's Special activation—thus improving Arthur's matchups against foes running Bold Fighter, Wrath, and other similar skills—makes it a neat option to consider over others. To consistently use it, however, a sustaining Special must be used to keep him above the skill's HP threshold.
• Quick Riposte makes up for Arthur's otherwise unassuming Speed at low levels of investment, offering him an additional follow-up attack in enemy phase. Like Guard, it also has an HP threshold and requires HP sustain to consistently use.
• Lull Atk/Def and Dull Close are also up for consideration as they nullify melee opponents' current bonuses. Whereas Lull Atk/Def is relatively difficult to acquire but further debilitates the foe's Atk/Def by -3, Dull Close is more accessible and also neutralizes Speed if that is relevant.
• Wrath allows Arthur to passively charge his chosen Special and get 10 additional true damage to activations when below 75% health. If using Sol or Noontime, this would help get additional damage in conjunction with sustain.

Passive C: Atk Smoke / Time's Pulse / Flexible

• To engage multiple enemies comfortably, Atk Smoke is best as it inflicts a harsh Atk penalty of -7 on surrounding enemies relative to Arthur's primary target.
• As Arthur's Axe does not feature a slaying effect (i.e. cooldown count -1), Time's Pulse can act as a stand-in for it. By reducing max cooldown by 1 at the start of the turn, Arthur can activate certain Specials more often especially with a Breath skill. One combination of note is a Breath skill + Aether + Time's Pulse, which allows Arthur to activate Aether within a single turn against most enemies.

Sacred Seal: Quick Riposte / Close Def / Atk/Res Bond / Atk/Def Bond / Swift Stance / Fierce Stance / Flexible

• If it's not run in the B slot, Quick Riposte can be used as the Sacred Seal to grant follow-up attacks in enemy phase, especially for builds not investing heavily in Speed.
• Close Def perfectly suits this build as it provides additional protection against both magic and physical attacks in close-range.
• A Bond or Stance of choice provides a neat amount of additional stats, depending where you want to invest in. Whereas Bonds offer superior stats, Stances make Arthur less reliant on positioning and less susceptible to skills such as Aversa's Night and Sudden Panic.

Preferred IV: +RES / -HP or -SPD

• Resistance is the one and only Asset for this build as Arthur is expected to tank multiple magical hits (which are basically the lifeblood of Aether Raids) in a single instance. 
• It is expected of users of this build to superinvest in Arthur in order to increase his stats and thus overall tankability. Inevitably, merges will neutralize the Flaw of choice anyway.

Weapon: Arthur's Axe (+Eff)

• In Aether Raids, debuffing structures and skills such as the Bright Shrine or Upheaval are commonplace. Should Arthur be hit by factors like these, the refined part of Arthur's Axe activates and grants an in-combat buff of +5 across his stats.
• Although Arthur cannot truly neutralize penalties like other units' unique weapons (Demonic Breath and Fimbulvetr in particular), both effects of his Axe ensure these penalties cannot truly cripple him in combat, though Panic is still something to exercise caution against.

Assist: Reposition / Smite

• Avoiding unnecessary combat an important aspect when duking it out in Aether Raids Offense, and Reposition helps with this by dragging supporting allies from enemy range by a space.
• Alternatively, Arthur can play support by using Smite to launch allies far, allowing him to test out traps safely and get your other units to a desired destination (such as enemy territory or an Aether structure) faster.

Special: Sol / Noontime

• Sol and Noontime are desired over other Specials as Arthur will often find himself engaging in multiple rounds combat in a single turn, making sustainability a priority. With Geirskögul or other similar support, Noontime is preferred more often than not due to more frequent activations, but Sol has higher returns against healthy foes.

Passive A: Distant Counter

• The vast majority of Aether Raids defense setups rely on the increased range of tome-, bow-, and staff-wielding defenders, making Distant Counter a required option especially for the purpose of omnitanking.

Passive B: Null C-Disrupt / Lull Atk/Def / Lull Atk/Spd / Quick Riposte

• Null C-Disrupt specifically counters enemies using cancellation skills, including Dazzling Staff, Firesweep weapons, and more. This ensures that foes cannot simply get a hit for free, get refreshed by a Dancer/Singer, and wreak havok on other parts of your team.
• For further ease of tanking, a Lull skill cancels out the current bonuses of enemies and punishes them even further by -3 in its respective stats. Atk-based Lulls are preferred to directly debilitate damage.
• Quick Riposte is a cheap and accessible alternative that offers a follow-up attack in enemy phase at healthy levels of health. Using this in the B slot also opens up the Sacred Seal to other options.

Passive C: Pulse Smoke / Atk Smoke

• Defense setups with precharged enemy specials (such as Galeforce, Blazing Wind, Glacies, etc) can be extremely deadly and pressuring, and there are fewer better individual solutions than Pulse Smoke, which increases enemy cooldown by +1 after combat.
• Atk Smoke is significantly more accessible and directly reduces the amount of actual damage done to Arthur on affected enemies.

Sacred Seal: Quick Riposte / Swift Stance / Fierce Stance / Distant Def / Atk Smoke

• If not run in the B slot, Quick Riposte can be run as the Sacred Seal to secure follow-up attacks against speedier enemies, thus closing matches more frequently.
• Enemy phase-based Seals, including Fierce Stance, Swift Stance, and Distant Def, considerably improve Arthur's performance in said phase. Depending on your playstyle preference, Fierce Stance improves Arthur's one-round matchups, whereas Distant Def and Swift Stance target key tanking stats.
• Atk Smoke can be run here if not in the C slot.