Any idea from Sonya -Spd +Atq?

Hmm, mine is +Spd-def and has Special Spira, but that's not a good idea. I would honestly keep her that way. There isn't much you can do given that her spd is so low .

Ideal would be to build for OHKOs using special spiral supported by infantry pulse. Death blow 4, atk+3 seal, glimmer. 61 atk and a glimmer+10 will KO a lot of things in one hit. Alternatively, L&D, blazing AoE special, heavy blade. C-skill could be odd atk wave, res ploy might be ok but it doesn't work against high-res units including most well-merged gen1 mages.

Build her like Lilina. Just go for oneshots and use Glimmer. Seeker knows