What to do with -Spd +Res Sonya

- spd might be better than neutral speed.
I'd go +spd first, then -spd. You could do any build julia can do like a Raven tome build. Her base moveset is pretty decent so you could leave her at that and use her on arena assault.

You can give her Quick Riposte due to her low speed or still go for G Tomebreaker.

Foddering her off though is another option, since her native skills are pretty good, Mirror strike and Res Ploy.

Sonya is pretty hard to IV screw outside of -atk. Just use the standard build, Dark Excalibur, Reposition, Iceberg or Glacies, Fury 3, GTBreaker 3, Res Ploy & Atk Ploy Seal. Don't view her as being in competition with Nino, her role is that of an offensive res tank for blue/green mages, not a full sweeper like Nino, so +res is a good boon, and even with speed investments she rarely doubles.

My Sonia has this exact IV and has been decent with QR2 as a B-Skill. She is probably the third or fourth green mage off my bench (Cecilia, Nino, Julia are my primary other options); but still useful in AA.

Gonna play the Devil's advocate here and suggest that, if you already have a good Green tome user, you give Res Ploy to someone like Sanaki. Or if you're feeling extra wild, Mirror Strike to Felicia for those F A T Glacies procs.