|Unfortunately for Ursula, her unspecialized statline and no access to IVs makes her a difficult unit to work with. That said, she does possess just enough in each stat to warrant a build that is... unconventional, to say the least.
To begin, Ursula's strongest stats are her speed and her resistance. And what are those two stats good for? Well, being a resistance tanker! Thankfully, as a blue tome unit, Ursula has access to the only Distant Defense tome in the game (as of Ver. 2.7.1), Blárserpent. In doing so, on enemy phase, Ursula will have 36 resistance, which is certainly nothing to sneeze at. And, refining her weapon with the resistance refine brings it up further to 39 res.
Her assist is simple in nature, draw allies back, thus putting distance between them and the enemy without compromising Ursula's position. That said, you can always exchange it for something else. Tailor it as you need. Her special, too, is fairly simple in nature. Her primary source of damage will be from her special, and Iceberg/Glacies can deal around 20/32 damage when conditions are right. A two-count special such as Moonbow may not provide the damage she needs to defeat foes, but as before, her special is at your discretion.
Her A skill complements her weapon. At 32 speed, she can be doubled by many, many units, and she cannot access a boon to mend that difference. As such, Darting Stance brings her natural speed up to 38, meaning a unit must have 43 speed to double her. That can be a difficult threshold to reach for some mages. Of course, cavalry buffs from allies will be crucial to her survival. If your allies can raise her speed well enough, you can choose Swift Stance to bulk up her resistance even further.
Dull Ranged, her B skill, acts both offensively and defensively. Because she is a ranged magic counter, she wants to ensure her foes are as weak as they come, due to her own insignificant stat pool. Nullifying buffs makes it far easier for Ursula to avoid doubles as well as dish out a little more damage.
Her C skill depends on your team's composition. She'll be right at home in a cavalry team. A cavalry buff makes the most sense, particularly Goad or Hone, as she wants make sure her offensive allies can deal with powerful foes. Of course, a defensive buff may make it easier for them to deal with physical threats for her. If you're running her on a mixed team or otherwise, a ploy, smoke, or tactic skill can work here.
Her Sacred Seal has a little flexibility. You can choose to give her Distant Defense to double down on Blárserpent (granting her a fantastic 45 resistance), or you can bolster her overall bulk with HP Res 2. HP Res 2 may be more efficient than Distant Defense if you choose to run a Ploy skill on her. It also worth mentioning that HP Res 2 will bring her physical soak to exactly 60, which should be just enough for her to survive one sword or lance attack at full HP. If you swear to never put her in way of a melee unit, Distant Defense can be used to ward off daggers and bows.