-Atk +Hp Sonya

Those IVs are... really hard to work around, especially given you have a better Julia. I'd say try like hell to pull for a better one if ever we get her as a focus again (even neutral), or use her as Res Ploy fodder (would actually work incredibly on Julia). But given my personal favoritism for Sonya (she's my S support) I'd say go with the former, and in the meantime put all that SP to use by giving her her skillset so she'll be all geared out once you get a better one (I had to do this with Delthea). In that case, go with the standard Fury 3/G. Tomebreaker 3/maybe Iceberg, Reposition, Mirror Strike 2, etc as skills to switch around once in a while as you have TONS of SP to burn.

If anything, fully gearing her out gives you at least a solid Arena Assault reserve unit, with Fury 3 she'll reach 47 atk. I suppose you theoretically COULD also build her as a Gronnraven user if you're committed to those IVs, but I don't recommend it.

I have a -atk +def Sonya and it's not that bad. I use her default kit with reposition and a breaker skill (I switch between different ones, bow breaker saved my ass more often than not !) and I still use her more than my +res -HP Julia.
It's not ideal for sure, but she helped me a lot with CC and squad assault already.