Gregor

Weapon: Blade of Favors

Assist: Positional Assist 

Special: Bonfire / Sol / Ignis

Passive A: Solid Ground 4 / Sturdy Stance 3 / Atk/Spd Finish 4 / Atk/Def Unity / Atk/Def Ideal / Steady Posture 3 / Kestrel Stance 3 / Spd/Def versions of mentioned skills

Passive B: Null Follow-Up / Atk/Def Bulwark / Lull Atk/Def / Spd/Def Tempo / Quick Riposte / Dull Close / Guard

Passive C: Joint Drive Atk / Joint Drive Def / Inf. Null Follow / Atk/Spd Oath 4 / Atk Smoke / Atk/Def Menace

Sacred Seal: Steady Breath / Null Follow-Up / Bonus Doubler / Atk/Def Form / Spd/Def Form

• Gregor's best overall build consists of skills that grant him the quintessential melee sword duelist effect loadout: damage reduction, quick/instant Special retaliation, Null Follow-Up, and a large helping of physical stat uptake. . 
  • Blade of Favors already provides several of these between its CD -1, 40% damage reduction on first hit, bonus/penalty-inducing effects, the blade further doubling enemy penalties with at least a 5 stat swing in Atk/Spd/Def minimum. With penalties correctly applied and additional passive skill investment, Gregor should have little trouble shrugging off hits from many physical foes.
• As a melee specialist build, it makes typical sense to invest in Gregor's relevant and stronger stats: Atk and Def. 
  • Solid Ground is already a pretty good starting point that does what the doctor asked, but offers no other combat effects beyond boosting Atk and Def. It also negatively impacts Res, which negatively impacts a few emergency Dragon matchups. 
  • In Solid Ground's place, you can run conventional tanky Atk/Def passives such as Sturdy Stance and Atk/Def Unity for additional combat effects. Ideal 4 is recommendable as Gregor can easily self-start it with the start-of-turn bonuses his sword gives him, but like Solid Ground only really improves stats. 
  • There are no Atk/Def variations of the Finish skill line at this time of writing, but regardless, Atk/Spd Finish acts as a method of providing extra damage and sustain on Gregor's Specials. A little extra Spd never hurts, either, as Blade of Favors already puts Speed matchups in Gregor's favor. 
• There's a few top choices Gregor can run in his B-slot thanks to his infantry access. 
  • Null Follow-Up is a standard for negating follow-up manipulation common in the game, but is also accessible as a Sacred Seal. 
  • Alternatively, you can run a few swing skills here: Atk/Def versions of the Bulwark and Lull lines enable Gregor to also heal himself or shut down enemy buffs respectively, and while the Spd/Def version of Tempo is the only relevant one, a little extra extra Spd, again, is always appreciated, and you can even inherit Atk/Spd Oath 4 from the same Shez (M). 
  • For budget options, Quick Riposte guarantees a follow-up on incoming enemies, Dull Close is an accessible but slightly inferior version of Lull, and Guard acts as a deterrent vs enemies Specials but may require recovery due to its strict >80% HP condition.
• As far as the C passive, considering that Blade of Favors already provides self-buffing and debuffing, it can be a little difficult to pinpoint good options that aren't Menace and Oath — though they're actually still pretty good for debuffing units that avoided Blade's columnar range and providing Gregor a guaranteed additional Atk/Spd +3 respectively. At any rate, Blade of Favor's combat effects activate if Gregor parks himself at least two spaces near some ally, so a Joint Drive rewards that directly. Atk Smoke 4 also runs into some redundancy issues here, but similarly helps apply penalties to unbuffed enemies while also adding follow-up denial to the mix. Finally, his base Inf. Null Follow is perfectly fantastic as a support option for fellow infantry units; crudely speaking, there's never enough Null Follow-Up right now.
• In the Sacred Seal slot:
  • Steady Breath combos best with a 2 CD Special — specifically Bonfire for reliable burst or Sol for sustainability —for instant retaliation on counterhit and tops off with a nice boost of +4 Def. 
  • Null Follow-Up, again, is just a great option to have in general to cancel various follow-up effects.
  • Bonus Doubler takes advantage of the self-buffs to Atk/Def/Spd that Gregor provides to himself, and generally provides more stat coverage than other Sacred Seals like Atk/Def Form unless a Lull / Dull effect is in play. 

Weapon: Blade of Favors

Assist: Positional Assist 

Special: Vital Astra / Glimmer / Galeforce / Bonfire 

Passive A: Atk/Spd Finish / Surge Sparrow 3 / Atk/Spd Unity / Atk/Spd Ideal / Swift Sparrow / Atk/Spd Solo 

Passive B: Null Follow-Up / Spd/Def Tempo / Spd/Def Bulwark / Lull Spd/Def / Frenzy / Velocity / Other Dodge skills / Wings of Mercy / Desperation

Passive C: Time's Pulse / Joint Drive Atk / Joint Drive Spd / Inf. Null Follow-Up / Atk/Spd Oath 4 / Atk/Spd Menace

Sacred Seal: Bonus Doubler / Flashing Blade / Heavy Blade / Null Follow-Up / Blade Session / Atk/Spd Solo

• A more independent, player phase build is also very effective thanks to the fact that Blade of Favors applies such a large Spd/Atk/Def differential between Gregor and successfully debuffed enemies. If Gregor is running a mixed-phase version of the build, a Far Save partner to account for ranged combat; Gregor's magic tanking still generally leaves much to be desired.
  • This helps significantly eases the off-putting 37 Spd compared to other Spd-heavy sword competition and allows him to improve on his natural offenses and follow-up performance/avoidance. 
  • The debuffs and Vulture effects also ensure that his Atk and Def remain consistently strong regardless of investment, so Gregor can afford to invest in his Spd heavily without too much concern for overhead. 
• The build's passives are pretty typical player phase unit fare: the main considerations are investing heavily in initiation-friendly, Spd and damage-boosting skills to push Gregor's natural offense far. 
  • The usual A-slots are listed, but Atk/Spd Finish and Surge Sparrow take special note as they significantly improve Gregor's staying power with healing upon Special trigger — Bonfire, Glimmer,  or ultimately, Vital Astra, are great and reasonably-balanced choices here. Their usefulness dwindles a bit with Galeforce, though, so you can keep other skills in mind. 
  • If you're not using utility skills such as Flashing/Heavy Blade for increased Special acceleration (highly recommended with Galeforce) or Null Follow-Up for general even ground, the Sacred Seal typically should be taken up by another stat-boosting skill. Bonus Doubler works best as it synergizes with the triple bonus of Blade of Favors but is vulnerable to bonus dulling effects and is otherwise open for other Atk/Spd skills.
  • The above applies largely to Gregor's B slot also, which should provide some sort of primary combat utility. 
    • Null Follow-Up is fantastic as usual here as getting Gregor to make natural follow-ups is of utmost priority. Having that outsourced somewhere else (like the Seal or from another ally), you can run the Spd/Def versions of Tempo, Bulwark, or Lull to increase damage and Spd while benefitting from anti-Guard, healing, and bonus cancellation, respectively. 
    • Dodge skills are par the course for fast swords, and Gregor and leverage the increased Spd investment by applying damage reduction on enemy hits. Notably, Frenzy, Velocity, and Close Call are preferred for having desired secondary effects like Desperation, the offensive half of Tempo, and giving Gregor a method of escapability, respectively. The damage reduction also applies multiplicatively with Blade of Favor's existing effect: under maximum skill conditions, the enemy's counterattacks will be reduced drastically by 64%.
    • Wings of Mercy and Desperation are lower-rung alternatives. The former is a warping option that works particularly with Galeforce strategies on Aether Raids Offense, and Desperation is good budget all-around as an accessible version of Frenzy, and an immediate follow-up allows faster Special activation and more seamless one-round KOes.
• Time's Pulse is simply generally useful as the C-slot of choice here. It lends itself to a variety of improved Special activation tricks, ranging from an always-readied Vital Astra (which is important considering this Special's damage reduction kicks in only if it's readied) and Glimmer, to less charges required for Bonfire and Galeforce. Aside from this and Inf. Null Follow to support the combat of other infantry allies, Gregor can run self-improving skills such as Joint Drives as well as Atk/Spd Menace and Oath 4 (with some bonus/debuff redundancy here notwithstanding).