I finally beat this. Some notes from my personal experience:
This is definitely difficult if you don't have the ideal units (ranged units who can kill the ninjas make this much simpler) but it's certainly doable without spending any orbs. I did it with 4* Olivia, 4* F-Robin, 4* Fredrick, and 5* Catria (Subaki should work even better). I think an equally effective team would be Olivia, F-Robin, Stahl, and Lissa but I didn't try it.
Related to the above, I found that when using Frederick as a tank, you can't use a healer; you'll need a blue unit to kill the sword cavalier. Frederick's Hammer is nice for the knight, though he doesn't one-round him.
I boosted F-Robin to 4* level 34 just for this. At that level she needed Olivia's Hone Attack to one-round Ursula. I would not recommend trying this without F-Robin.
Right after she killed Ursula, I found that moving F-Robin left & up to attack the knight through the wall was a safe move because the ninja would heal the knight instead of killing F-Robin.
This map gave me quite a hard time and took 5 stamina potions.. I am already leveling Ursula in case Xander's map is also cancer.