Sona is one of the best supports by late game. My personal bias goes towards Janna for best support late game as she totally owns team fights which is very common at this point. That doesnt mean Sona is useless. A sona who can utilise her power chord correctly can win game for the team. I seen many sona's use power chord after the q in team fights. That is totally the wrong chord. Honestly you don't need to damage but instead need to do something to protect your team e.g. power chord after w to reduce damage output or power chord after e to slow down target.

Not taking song oc celerity at all by level 4 is really silly. The slight movement speed can save you from ganks. How so you ask? you run faster hit them with a power chord and the person is slowed. Ganks are very common at this stage.

I guess this isnt perfect at all but really i never saw a effectiv late and mid game support sona. And i played many times like this with sona and win at 80% (if teammates arent noobs).
And when i start i dont need health pots or so. I have my heal skill.

Heal and Flash is my standart since lv 16 and it is astonishing how often this safes my ***.

That Power Chord is powerful i said.

This is pretty weak as far as an AP sona build goes. Also, I have no idea where you get that she "does no damage" and "dies almost instantly" late game. In my experiences with Sona, I am never either these unless I've D/C'ed.

Heal is an awful summoner to take on her, as you give up CV which, in my opinion, is a must have spell on a team. She already has a heal on a 4 second or less cooldown, you should not need to this as a summoner slot.

You open with a meki pendant and mana pots, which pretty much cut off any sustain you could have had through pots, or vision through wards. Also, you never upgrade the meki into anything... The flat mana runes puzzle me, I don't know why you wouldn't just use Mp5 runes instead. If you are spamming your Q and W to the point that you need to use mana pots, you're doing something wrong, or your laning partner is a suicidal idiot. Either way, something isn't right.

Sona is so much more than stacking ap and mashing Q and W. Her powerchord is an incredibly powerful tool, useful for a massive poke early, and then a debilitating slow while chasing, and a strong damage reduction on the enemy carry in teamfights. She relies heavily on positioning, and is incredibly rewarding if played effectively. I've rolled games as her without building a single AP item, and still end up with tons of kills and assists. Even as far as an AP Sona faceroll guide goes, this could use a lot of work.