How should I build a -spd B! Tharja?

Thats a terrible iv I feel bad for you, but there are some skill sets that will get her speed back! I would prefer life and death over fury, but the penaltys of life and death and her really low defence and resistance will make most enemys ORKO her. You can even use swift sparrow to further increase her attack and speed, but the disadvantage of that is the skill is premium so you probably won't fodder someone like Ayra, SM!Erika etc. Or maybe you can stick with her default a-slot skill in exchange for the horrible penalties that Life and death gives.

Desperation is ought to be the best b-slot skill for her as this will mentain her survivability at low hp (in the player phase) with the aid of reciprocal aid. I don't think anything else can match desperation. (If you had any other thoughts, you can decide.)

In my opinion, I think her c-slot skill is flexible, but I'd recommend a debuffing skills like threaten res or atk
Or she can have a hone skill that synergies with her default duo rally assist. As well as being -spd she can have the new smoke speed seel which will decrease the chance of enemy's doubling you, even though she can still double some units.

That's it I think. I mean you don't have to follow this if you have anything else in mind.

I know your pain, I am working with one +Atk -Spd

-Múspell Fireposy
-Moonbow
-Support flexible
-Fury-3, for some more stats
LnD-3, for go with a even more fragile glass cannon
Atk/Spd Bond/Darting Blow 3 if you want to use She but not invest to much.
-Swordbraker-3, for fast sword user...but with DC user will be issues
unless you use ardent sacrifice/reciprocal aid as support, Desperation-3 cant be a good option(worst if you put LnD3), 35 base spd is not optimal but can be used at least on middle Spd units
-Flexible, but Threaten Res or Spd can help
-Spd+3 Seal, will bring base spd to 38

Basically a "normal" Tharja with other weapon