Zelgius

Preferred IV: +ATK or +SPD or +DEF

• Since Zelgius isn’t suited to take the heat of heavy magic damage in Aether Raids, this set primarily focuses on boosting Attack and Defense to protect ranged units from physical threats and ideally one-shotting them in retaliation. 

Weapon: Alondite (+Eff)

• Although the inherent Distant Counter might go to waste in this circumstance, the refine for Special cooldown acceleration as well as +4 to all stats makes Alondite the ideal weapon.

Assist: Swap / Pivot

Special: Black Luna

• The choice Special in making sure your opponent is removed in a single strike regardless of their physical bulk by removing 80% of their Defense. 

Passive A: Atk/Def Unity / Atk/Def Ideal / Sturdy Stance 3 / Fierce Stance

• Enemy Phase-oriented skills that boost Attack and Defense are the ideal choice here. Atk/Def Unity makes for an especially effective option, as it can have Zelgius draw the attention of debuffs away from his allies while boosting himself and respecting the same condition as his Save skill. 

• Don’t have any of these fancy tier 4 skills? Well, Zelgius’ native Fierce Stance will do just fine in the meantime. 

Passive B: Special Fighter

• Provides the Special cooldown acceleration needed in order to instantly retaliate with Black Luna as well as Guard. 

Passive C: Near Save skills

• Near Save skills, ideally Atk/Def Near Save are the key to this build, as they allow Zelgius to enter combat in place of any allies within 2 spaces against melee foes. 

Sacred Seal: Mystic Boost / Atk/Def Solo / Sturdy Stance

• Since Black Luna is a purely offensive Special, Mystic Boost can provide a bit of the healing that Zelgius may need after combat. 
• Any skill that further boosts Zelgius’ key stats of Attack and Defense makes for a suitable choice of Sacred Seal.

Preferred IV: +ATK or +SPD

• In this case, matching Speed with your opponent to avoid being doubled becomes a lot more important.

Weapon: Alondite (+Eff)

Assist: Rally+ skills

• Any form of maximum SP skills including Rally+, Rally up, or alternative skills such as Harsh Command+ make for suitable assist skill options. 

Special: Black Luna / Aether

• Naturally, Black Luna remains the best choice of Special on account of its SP cost as a personal skill. However, regular 500 SP Specials such as Aether serve as valid alternatives. 

Passive A: Unity Skills / Sturdy/Mirror Stance 3 / Bond skills

• Since most mages aren’t built for 

Passive B: Warp Powder

• Is it a shame to use such an underwhelming skill for the sake of maximizing scoring potential? Yes it is but you might still find some use out of the additional movement options provided by Zelgius’ personal skill, Warp powder. 

Passive C: Save skills / Armor March / Armor Stride

• Being free to focus more on Resistance in arena, you’re also free to make use of both close and distant save skills. 
• If you’re using more than one armored unit, Armor March in the seal slot will provide a ton of utility in boosting the movement of Zelgius and his adjacent allies.
• Alternatively, you can have Zelgius boost his own movement with the solo condition of Armor Stride.  

Sacred Seal: Mystic Boost / Armor March / Max SP stat boosters

• If not already used in the C slot, Armor March can also be used here. 
• Any form of maximum SP simple stat boosters such as Atk/Def or Atk/Spd solo also make for solid Seal slot options.