Celica +Res -Spd. What should I do with her?

I say fodder her off unless you really wanna keep her. If you do just give her a +3 speed seal and some A skill that boosts speed, something like fury

Distant Defence fodder or run her something like this,

Weapon: Ragnarok
Assist: Reposition / Flexible
Special: Iceberg / Luna
A-Slot: Distant Defence
B-Slot: Renewal / Quick-Reposte
C-Slot: Flexible
Sacred Seal: Distant Defence

However, if you want to build her normally you could just do Life and Death / Death Blow + Renewal / Breaker Skill and call it a day. Celica prefers to OHKO most of the time anyways and she’ll still hit like a truck with this build.

Another options is Fury!Vantage. This way you can fix her speed and capitalise on her boon for the first fight and then transition to Vantage for safety. When ran with a healer, who have become pretty strong and viable, makes this build quite nice.

I have a +res -spd Celica as well. I know that feel :')
Here's my +Res -Spd Celica build:
Weapon: Ragnarok
Assist: Draw Back
Special: Moonbow
A-Slot: Life and Death 3
B-Slot: Desperation
C-Slot: Threaten Res
Sacred Seal: Speed +3

I was trying to make Celica works by giving her darting blow back then, but it doesn't work the way I wanted.

I decided to go with her usual kit, LnD. with summoner supp and speed seal, she reached 40 speed. with Ragnarok it becomes 45 unbuffed. thanks to IS listening to my tears(releasing S skill and support system), I managed to fix her :D

for her B, either Renewal or Desperation could work. Right now, I'm using desperation because I don't have a Renewal 3 fodder atm

moonbow is good for her special, as she can get more KOs against unit slower than her