|Sully, the ultimate forgettable unit. Having a poor statline to prevent her common use but lacking any sort of meme culture like other units such as Odin or Wrys, she receives virtually no attention. That said, she does have potential running a mixed bulk set, though her true potential resides in her synergy with her teammates.
Sully has serviceable speed stat of 34. While by no means spectacular, it is worthwhile raising it with a boon to get her to 37 Spd. This will help keep her from getting doubled by most units, with most of those who can do so more easily being sword units (such as Karla, L!Ryoma, and Mia). The only true threat to her in this regard would be Nino: Pale Flower. Because you want to maintain both of her defensive stats, the HP bane can be taken. She has a decent HP pool, so she can part with a little HP, especially when her weapon refine will make up for it. Her weapon refine grants her +4 Def, so she can better combat melee threats.
Her weapon and special are designed to help maintain her bulk and mend her poor attack stat. With only 40 Atk, her damage output will be pitiable. That said, she should be primarily dealing with red and blue units to overlook it. The Harmonic Lance will grant +10 damage and virtually guarantees 5 HP healed when Sol triggers. Should you prefer her to be dealing meaningful damage, switching her special to Moonbow or Luna may be of better use to you.
Her assist skill is 100% flexible. Personally, I find Reposition to be the best because of its high movement utility.
Her A skill, Close Def 3, raises her defense and resistance to a solid 34 when attacked at melee range. With her 37 Spd, it shouldn't be taking too much HP from her. Her Sacred Seal can either be Close Def 3 or Distant Def 3, depending on whether or not you would rather stack her melee tanking or give her some ranged bulk as well.
Her B skill is fairly flexible. In fact, your team's composition will likely determine the best B skill. It should either focus on reducing the foes' Atk or Spd to reduce damage as much as possible. If your allies grant speed buffs to Sully, reducing Atk may be better. Otherwise, a speed reduction should go here. A Chill or Seal skill would work perfectly fine in this slot. You may also opt for Guard, to prevent special procs from severely chipping Sully.
Her C skill is flexible as well. Like with her B skill, it depends on your team's composition. Should you prefer to make Sully more of a standalone unit, Atk Smoke or Spd Smoke could fit here. However, if you want better synergy between allies, giving her a cavalry buff works wonders. Since she is a tankier unit, she would be best put with Hone or Goad cavalry, as she wants to support her damage-dealing allies as much as possible.