Greetings plebians, gods and humans alike, here I present you with a guide to a very under-rated champion that can rack some surprising ELO if played well.
Call me S, call me Yuki, let us begin with explaining the might of AP Sona Mid.

Pros

Very unexpected(Who the frog plays Sona mid?)
Insane Damage post-Sheen
Simple to play
Can contribute even when behind
Useful in mid and late game
Ult decides teamfights
Overall has solid matchups
Has flexibility build-wise
Reliable both mid and support, so you are not useless when filling
Mobile with E, not to mention that it scales with AP
Solid array of skins(DJ and Guqin being my favs)
Best Tits in game(it had to be said)

Cons

Squishy(imagine a Kog'Maw with Tits)
Weak vs Assasins and champs with High Range
Mana Hungry
Teammates might get cynical in champ select. Don't be toxic for even a second

The Mastery set-up I use is pretty obvious, so I'll just touch up on Stormraider vs Thunderlord

Stormraider is good early, when landing 3 hits quickly is not easy. You proc it easy and you run away even easier. Could be helpful vs dangerous junglers. Can be useful, but I prefer it on Nasus and Veigar

Thunderlord is the usual choice. Ult-Q-Auto procs it for even more secure oneshots

You can run the classic mage set-up if you want, my set-up is a manner of preference from my Nasus days

Here I will touch up on the optional items, since the standard build is self-explanatory

Athene can be useful when behind or if you wish to take a more supportive role whilst still dealing nice damage

Tear-Archangel. Personally I think it is too risque of an investment to take it. Takes too long to ramp up and leaves you with low damage and defences in the meanwhile. Although it can prove its worth late game

Frost Queen. Mini Stormraider and Gold Income, plus you don't have to facecheck. Try it out, but don't buy Spellthief and Frostfang, you'll just screw yourself over

Gunblade. Can be fun, provides sustain, small burst and helps with last-hits. Give it a shot if you want to

Mejai if you are fed. Nuff said

Rod of Ages, run it vs Zed, LB and other dangerous midlaners, although always get Sheen first

Banshee if the spellshield proves crucial or if you don't run 10% CDR runes like me.

Experiment and decide for yourself, I prefer the balls-to-the-wall oneshot build provided above

Always max Q, combined with Power Chord it chunks early and will make the opponent regret laning vs you
E is a one-point wonder so max W second
Max R when possible

Laning:
Poke with Q,
Last-hitting is tricky but doable
Sometimes it is better to land a q than to risk getting nuked by going for the auto
If the opponent has <300 HP, at lvl4+ you may be able to kill if they don't have any gimmicks to stop you
Post 6 you either dominate or cower in fear. In case of latter, roam. In case of former, kill then roam
Mid-Game
Go Mid, kill their mid, roam. If possible save up ult to secure a double kill botlane
Late Game
Stay in the back and always watch out for predators. You get nuked easily. Otherwise, unleash hell

So you've reached the end of the guide? Congrats! If you feel enticed to play your opponents like your Etwahl, feel free to do so and I hope you rock as well as I do. Just don't pick her vs me xD

Good luck summoner and enjoy your freelo!

Math to explain the damage
Why Zhonya, Rylai and others are pointless
More pictures
Highlights
Full Gameplay videos
Elaborating more on certain topics
AD and Hybrid Sona Builds
More music and Tit jokes :)