Pros

E is a good CC ability in lane.
W provides long range vision/slow/damage.
R provides team fight capability.


Cons

Not very good against hard engage
Not very good if you can't land your E (or opponents are good at dodging)
You have some peels but they are not 100% reliable for protection
Your ADC freaks out on you in champion select.

Glacial Augment: Provides slow after a successful E pull. This is also good vs melee enemy support. Furthermore, your Twin Shadow slow with this provides insane utility and will let you land your W only the target all the time after the ghost.

Perfect Timing: Very important for the cooldown reduction on Zhonya as you will need it to survive everytime you Ult for the teamfight. The free stopwatch is always good.

Biscuit Delivery: This is mainly for sustain. Swain base health is low and will get poke down quickly if a mistake happens. You can always sell this if your lane is easy.

Cosmic Insight: Mainly for the Item CDR.

Cheap Shot: This rune provides you enough damage with your passive on the CC target. Another good choice is Ghost Poro if you are good at placing it.

Ingenious Hunter: This rune is very important as it will allow you to have very short Twin Shadows as well as Zhonya for every team fight.

Basic:

So the important thing about Swain support is to use your E to catch the enemy for possible kills. You can create a lot of pressure in lane standing beside your minion waiting to pull the enemy champion. The best thing about the Swain's E is that it can go through the enemy minions to pull. Pay attention for when the enemy champions walk up for the last hit to land your E on them and pull them in. Land you W on the pulled target for slow and damage as a combo.

The Vision of Empire(W)

This is a great ability once you learn to appreciate what it can do from a far distance. In game, you want to use it for the following purposes:

1. Scout the river bush when no vision (also good to use on Dragon to check).
2. Use on enemy champion you hit E for the extra damage and CC.
3. Try to use it on possible enemy recall location for interrupt or even possible kills.
4. Use on fleeing enemy by placing on their flee path to help secure a kill for teammate or simply slow them down.
5. Use on the engaging path of enemy (especially enemy jungle) to disengage.

In lane in most cases, you want to E first before you W. However, it is possible to use the W to zone the enemy path to help landing the E.

Team Fight

Keep in mind you are a support so you are not that tanky until you have Zhonya. It is important to not dive deep and die before you prove your worth. In the early phase, you want to stay around your ADC so you can R, E, W the enemy champion that is trying to dive on him. Stack on the ADC with R has the potential to help enemy misclick the damages on you instead. If you can't stack on your ADC, use the E and W to peel. If your ADC does not require protection, stay with your front line with your R and use your W for slow on the fight and E to pull the enemy backline for mis-position.