Best build for -Spd +Def Celica?

I'd probably run swordbreaker to make up for her -Spd bane.

Alternatively, keep her weapon and give her Fury or Swift Sparrow. Stick Speed+3 seal on her and she'll hit 41~42 Spd on her first initiation, which isn't bad by any means.

Failing that, you can always fodder for Distant Defence or wait for one with better IVs.

You can also try for a blyn check:fury bowbreaker set
She gets 39+28 def for 67 physical bulk so she cant get one shot by blyn. With it she can take on slow dc units with moonbow/glimmer.

Her special and sc are flexible.
Atk+3 so she can ko blyn without special procs so she can charge up luna instead.
Spd +3 so she has better matchups overall
DD only helps if ur trying to take one tharja or nino basically