Hey guys thought I would write a guide to explain how to play swain middle correctly, and win lanes very easily. I have a 70% win rate in EUW on mid swain, and have beaten the likes of xpeke on zed. IGN:Ceren
Swain is normally considered a top-laner however, his kit is well positioned about defeating melee champions in mid-lane.
So onto the guide....

The runes are standard for mid-lane, focuses on exploiting your mid and late game power, no MR or armour because it is pointless if you able to dodge skill shots and with your defensive masteries work much better to spiral into been unstoppable.

I know I will get flack for this but, ignite is pointless in mid-lane on swain. This is purely because of you kit, so let me explain.
Your e increases your total damage to the champion affected. IGNITE ISNT AFFECTED since it is true damage. Exhaust on the other hand though reduces the champions damage to you as well as their RESISTANCES. This stacks correctly with your e so you do even more damage.
This is the whole reason that swain dominates fights against champions such as Zed and Ekko.

So start Doran's in every lane, there is no exception to this rule regardless. Even though you may disagree, this allows you to constantly push and harass your lane opponent with your passive mana restoration as well. Once you loose your ability to E+Q and use W strategically you will 100% loose lane.
Afterwards go for catalyst, if you cant afford it but have an advantage in lane, push it by buying 2 more dorans ring and boots.
ALWAYS HAVE 3 POTS AFTER FIRST BACK AND A WARD. I cannot stress this enough. With your low mobility ganks will be the main way you will loose after been poked down.
Heading for RoA then Hourglass is the optimal route so don't build anything else. Afterwards build Deathcap then Void staff to become a late-game god.
DO NOT BUILD FROZEN HEART OR SPIRIT VISAGE. For the love of god DO NOT, you are not a top-laner you are a mid-laner, late-game you will be tanky but you will not have the impact without the damage items.
Last item is dependent on the enemy team and whether you are loosing or winning, just comes with experience to know how to finish.

I know this guide may not be totally complete or you may not agree with it, but feel free to leave feedback and any question that you would like answering, and I will get back to you as soon as possible.

Cheers