Sona mid IS VIABLE! :O
I carry like every time i play sona mid. Even against broken champs like Aatrox and ivern!

All you have to do is use your passive offensivly. Build AP Everywhere & Full CDR. Keybash & use ur Q whenever u can to activate ur passive. make sure ur using ur blue passive thingy cuz it does most damage. u can also ult and ignite.
Once you build
it's literally impossible to escape ap mid sona once she sees you.
Unless u flash, then ur not fked.