This build is good for one shotting and keeping your team alive. If you get caught out you can speed away with your E and few champs will be able to keep up. On the flip side you can chase the majority of champions with ease thanks to your E and high range on Q's.

The main con with this is that you are squishy. If you don't pay attention and a high burst champ such as Akali jumps on you, then you can get insta bursted. You can still win against high burst champions though thanks to your heal and high damage.

During team fights with this build you can either be in the middle dealing damage to the other team, or you can be on the outside of the fight healing your team and giving massive pokes with your Q's and empowered auto's.
Always in team fights watch your team's health. If you need to help your AD Carry escape use your W to heal and shield them and your E to guide them to safety.

This is the most difficult part of AP Sona. Her normal AA just doesn't do much damage making csing difficult. You either have to pay close attention to that, or do your best to get what you can and keep the enemy laner away from their farm. Depending on the opponent I will actually stand on the opposite side of my farm and be focusing more on attacking the enemy that way they can't farm either.

With your late game CDR, it becomes easy to just spam your abilities during fights and not have to worry about your mana thanks to Seraph's and Morello's (I also try to pick up blue buff when I can). This means that you can make practically every other AA an empowered one dealing bonus damage.