Its actually quite simple on how to deal with an AP Sona, watch them, they have to get in to an adc's auto attack range to poke with their Q, and that Q deals low damage. Its the Q + Powercord that hurts.
She does have some fast cooldowns, but all you need to do is just wait out her mana pool. To keep it topped off late game it would require 50+ mana per second and most AP Sonas wont get mana regen because it lowers their potential burst damage. The ult is really easy to dodge as she has to get in range for it and that's really noticeable.
Late game, an AP Sona with lich will hurt, but here is the thing. She is extremely squishy, and if you stay out of her ult range, you will kill her. Catching an AP Sona is quite taxxing if you have no gap closers, but she is light years away from being as fast as fucking Janna. Chasing an AP Sona will destroy her mana pool.
If you are melee, get a little bit of mr, A Sona that builds magic pen is very rare. Many people don't build mr and bitch about ap poke. Its fucking burst, of course its going to do damage if you don't build against it. I mean if the enemy had a Le Blanc, you would build a little bit of mr wouldn't you?
Now as for Utility Sona's, they are WAAAAYYY faster than AP Sona players. They don't build magic pen boots.
Even though they lose out on about 75% of the damage, Utility Sona are the ones to beware of in a chase. They have high mana regen. They might not have the poke an AP Sona has, but they are faster and can spam their spells for minutes with that 40-50 mana per second regen. Utility Sona's can be as fast, if not faster than some of the normal Janna builds.
It is best to have some gap closer to take them out, but utility Sona's are also quite tanky if they build right so it might not be wise to chase alone.
She may be easy to play, but she is not exactly one that is easy to master, as I see many people who claim to have mastered her, but they have horrible positioning, get punished often when they go in to poke and they do not take in consideration their range.
Take advantage of that and you will stomp a Sona in to the gound.
The moral of this is, If its an AP Sona, you can 1v1 them if you have a bit of MR and/or dodge the ult.
If its a Utility Sona, be careful when you are alone because they can dish out more damage over the chase with out running out of mana, they can cast around 5-6x more spells with the same amount of mana than an AP Sona can and i recommend two people in that chase.
, oh your a melee, how cute...