I would say it depends on her B skill.
Special Fighter screams +Spd to me.
Crafty/Slick/Wily Fighter have an easier time with +Res.
At max investment (which is your case) I think her best set is:
+Res IV, Sturdy Stance 3, Slick Fighter 3, A/D Near Save 3, Steady Breath.
Let me explain why: her prf’s Res check means she must always have penalty neutralization in order to not lose it when debuffed, but she also wants penalty neutralization on Atk/Spd/Def, meaning Unity skills (which only give 2) are out of the question, and instead go for Slick fighter.
The also must-have guard effect then goes for the A slot. Having already a defensive NFU in prf’s refine and auto-double on Slick, means her most desired stats become Atk and Def, which explains Sturdy Stance and A/D Near Save. The breath effect is a given in SS, probably with Noontime to sustainability or Ruptured Sky.
With this set her match up against F!Edel is always consistent and she has the most defensive potential against other PP melee units like B!Eirika.