Holst

Weapon: Warrior's Sword

Assist: Positional Assist / Premium Rally+ Assist (for Coliseum)

Special: Vital Astra / Noontime / Glimmer / Ruptured Sky / Galeforce

Passive A: Atk/Spd Ideal / Surge Sparrow / Atk/Spd Unity / Atk/Spd Solo / Atk/Spd Clash

Passive B: Null Follow-Up / Spd/Def Bulwark / Atk/Def Bulwark / Velocity 3 / Other Dodge skills / Lull and Tempo versions of aforementioned skills

Passive C: Atk/Spd Menace / Atk/Spd Oath / Spd/Def Menace / Rouse Atk/Spd 

Sacred Seal: Bonus Doubler / Flashing Blade / Darting Breath / Blade Session / Atk/Spd Solo / Atk/Spd Form / Swift Sparrow 

• A simple, all-purposes combat build focusing on bolstering Holst's high Atk and Spd suits his equally simple weapon. 
• A premium Atk/Spd-boosting A-slot and Sacred Seal to further build on Holst's great offense. 
  • Surge Sparrow is a standout recommendation due to its recovery effect on Special trigger, allowing Holst to better maintain himself above Warrior Sword's minimum working HP threshold. Holst's default Ideal rewards him for simply abiding by said weapon's visible bonus power-ups, and its fourth variation has a high ceiling under optimal conditions. Finally, Unity prevents Panic effects from reversing Holst's bonuses and consequently screwing his combat totally over.
  • In particular for Holst, the Bonus Doubler Seal repeats Warrior's Sword's doubling effect twice over, allowing him to build up himself on every front assuming he's granted a visible bonus in all stats. If you don't want to be doubly vulnerable to bonus negation effects like Lull skills, though, any of the conventional skill boost Seals apply alternatively.
• As an infantry unit, Holst gets access to a nice variety of exclusive B-slots. 
  • Above all, Null Follow-Up is just a great choice all-around as it gives Holst more significantly favorable matchups than simply flexing his big stat muscles. 
  • Else, the Bulwark skill line allows him to more carefully keep himself above his weapon's working 25% HP condition due to its after-combat healing, plus they grant more stat dominance and enemy movement blockage.
  • Lull and Tempo skill variations also increase stat dominance while offering enemy bonus negation and anti-Guard effects, respectively.
  • Spurn, Velocity, and the others in the Dodge skill lines fortify Holst's general survivability with damage reduction, and he will likely get their maximum amount of reduction in most cases with his highly-invested Speed. Velocity, in particular, also conveniently comes half-packaged with Tempo's important anti-Guard effect.
• Holst's C-slot, in particular, matters quite a lot on this build. Seeing as all Warrior's Sword needs is visible stat buffs to function at maximum performance, it makes sense to feed it with premium, self-buffing options. There are a bunch of great options here at this time of writing; an Atk/Spd-based version of the Menace, Oath, and Rouse skill lines will be totally optimal in increasing his two most important stats here. At worst, do not settle for anything that boosts less than Holst's default Oath 3. 
  • As an important note, also make an attempt to provide bonuses through other ally-provided means to make full use of Warrior's Sword's stat buffs, such as premium Rallies, low-rarity Tactics, or other specialty buffing skills.
• For Specials, Vital Astra is definitely best for performance and Colisseum scoring as it bites back with Holst's highly-invested Spd in a short amount of cooldown. Else, Holst is perfectly fine with any of the low-rarity 2 CD choices; Glimmer typically gets high returns with Holst's great offense, and Noontime's healing keeps Holst and his weapon's effects around for longer.
• As a side note, it might be good to be mindful of which phases you would like Holst to specialize in. As a particular example, Vital Astra's firing can be made ready either exclusively by Flashing Blade in the player phase, or Darting Breath in enemy phase. Surge Sparrow is also an A passive that pretty much pushes Holst to mainly initiate rather than wait.
• Atk/Def versions of all relevant Passives may also be taken over Atk/Spd for a generally more physically tougher Holst. This serves to stand up against other melee enemies significantly better while still maintaining a reasonable amount of less invested defensive Speed.