My recommendation is to ask your team mates to go on mid. Swain can also lane, but he is a great character for 1on1 as well, and he works really great on mid. While doing 1v1 on mid, my advice is keep farming in the beggining and dont use abilities until you reach lvl 4.

As i recommended, I suggest leveling up Torment first, since the good AP scaling (0.8 AP ratio) & a lot of damage from skill (235) over 4 seconds..that means 58.75 DPS ..
After That , i suggest leveling Nevermove to lower its cooldown, and have more damage with it ..0.7 AP ratio & 240 damage from skill..which is pretty high..and the last one i suggest Decrepify , because 0.3 ratio only, and 85 damage per second at last rank, for 3 seconds, so 255 damage in 3 seconds..it is higher than Torment damage, but Torment Gives damage amplification too so ..thats why i suggest to lvl it up first :D plus Decrepify is channeled, if u go too far from target it will cancel..

At level 4, you can try using the following combo .. Hit Nevermove then Torment and then Decrepify . Assuming its a squishy char, he should be under 50% with this combo..If you are against someone hard, use this combo a few times , or keep harassing him with Torment until he is half hp or something.

After you hit level 6, you have 2 choices. If you are very experienced with Swain, and are pro in hitting with Nevermove , go ahead with this combo : Hit Ravenous Flock then Torment then Decrepify and then hit Nevermove . This combo does the best burst of damage from most of Swain combos.
If you aren't very experienced in hitting with Nevermove , try this one : Try catching the enemy with Nevermove , then try going fast in ultimate ( Ravenous Flock) , then use Decrepify to slow the enemy because the snare should have went off until now, and use Torment to finish him off if he tries to escape or he is low hp.

This is what damage you should do if the enemy had 0 magic resist with full build at lvl 18 :
Decrepify - 212.2 damage per second for 3 seconds => 636.6 damage in 3 seconds.
Nevermove - 536.8 damage.
Torment - 574.2 in 4 seconds => 143.55 damage per second.
Ravenous Flock - 174.8 damage with each raven.

Remember, thats only with full build and with enemy with 0 magic resist.But, with this build , you reach 54 Magic Pen at full build at lvl 18, so if the enemy has only the 30 magic resist from base magic resist, you'll do way more damage.Or if enemy has more magic resist, you'll do less :D

Around lvl 6, Swain starts to lose mana very fast..that's why i recommend you highly to ask your team mates to help you get blue buff around lvl 6 ..if you can get it, you can happily gank some lanes for some kills.Blue buff is just one of the best things you could have. Just like Anivia , Swain is one of those chars that love the blue buff..Even before a gank in late game, go ahead and grab the blue buff so you can stay in Ravenous Flock longer, and have lower cooldowns, it helps a lot !

The Core items are :
- Rod of Ages => Provides great survivability, some mana and ability power as well !
- Sorcerer's Shoes => Best boots available for Swain.
- Rabadon's Deathcap => Best Ability Power item in the game.More damage on abilities, more heal from ultimate.

Optional items :
- Abyssal Mask : Because most of the teams have at least 2 casters..so gives a bit survivability against them, and gives ability power too.
- Mercury's Treads : If the enemy has lots of CCers, those boots are your main choice :)
- Will of the Ancients : If you are doing well, why wouldnt you want to heal yourself more with ultimate and get some nice health points from your DoTs ?
- Archangel's Staff : More Mana, More Ability Power. Should be built first if you want to stay in your Ravenous Flock a lot because you can profit from it more.
- Zhonya's Hourglass : If they jump on you, pop the active from that item. 2 seconds immunity is enough to save your ***. You can also profit from it from the Ability Power (100 !!!) and from the Armor, which is a lot of AP & Armor.

If your team didn't bother to pick a tank and you want to go a bit tankish (I don't recommend that) , you can also pick those items :
- Thornmail : Assuming the enemy team has 2 carries AD, or only their ADs got a lot of feed.
- Force of Nature : Gives Magic resist, health regen, against many casters.

Pros :
- Really powerful in team fights, or 1v1, due to nice AoE abilities and DoTs to concentrate on a carry.
- Nice survivability
- Pretty nice burst of damage if combined with ignite (Not even like other casters, but nice as well)
Cons :
- Grumpy Old Man
- If another burster on mid, like LeBlanc, Anivia or Annie , if the other is farmed, and he catches a burst on you, you can't heal so fast with ulti and she will kill you.

My Recommendation is to take Flash & Ignite . Why ? Because of that : Flash is my favourite summoner spell. Can Flash to do a gank and pop Nevermove , can Flash out of a fight, through a wall..You can do a lot of things with it. Ignite : If you got against someone with full Lifesteal, or with high spell vamp, this spell is your friend. Or, if you go on an enemy 1v1 and put full DoTs on him, put Ignite as well and you can simply run if he is low HP and you as well.Those DoTs should finish him off.

Other good options :
Exhaust - Always good on any char.Lowers his damage, makes you able to hit easier with Nevermove.
Teleport - If you use that spell on every char, its good as well on Swain, to save a lane by defending it.
Clarity - Not best for Swain, but in early game you can still use it.
Ghost - Not bad, you can catch a running opponent or you can run as well from someone while low HP.

What NOT to take :
Smite - On Swain ? Really ?
Heal - Noob Spell ??
Rally - What the hell does that spell even do ?
Cleanse - While stunned or taunted, your ultimate Ravenous Flock still heals you, so no need to use that.
Fortify - That's what tanks are for -.- .
Revive - Noooooo...
Clairvoyance - Support spell oO ..

While ganking, my advice is to do it like that : Assuming you have high mana, staying in Ravenous Flock like 10-15 seconds isn't a big deal, so use Torment & Decrepify on the main targets to finish them fast.My advice is to use Nevermove only to save an ally if he is being followed by a melee carry, or you can just use it on a melee to disable him for 2 seconds from the gank. It can be used in many moments, so save it for the right time !

But, if you are sure u can finish someone really fast, another good try is to just jump on someone with Ravenous Flock activated, and Nevermove him and DoT him.Popping Ignite and some damage from your ultimate is more than enough to finish a carry..because remember, you are not focusing the tank !

If you don't want to go mid, or your team has a Karthus or another character dependent for mid, it's alright. You can also lane with Swain ..Here are the best combos I've found with him yet, on lanes :
- Taric - Very good char with that stun, so you can land your Nevermove without problems, and provides a bit of healing as well, in dramatic situations.
- Pantheon - Nice stun and some damage that you can finish someone easily.
- Malzahar - Not really recommended unless you find a good Malzahar . If you focus someone and Malzahar uses his ultimate on someone while all the 3 DoTs are up on the target ( Malefic Visions , Torment & Decrepify ) , u can simply use Nevermove after the Nether Grasp because u will land it for sure. Enough to gank a squishier on your lane that can screw the game if farmed a bit.
- Rammus - Taunt, damage, can take someone off of you, great char.
- Cho'Gath - Many combos, either you Nevermove first, either he Ruptures first, you will kill someone for sure. Plus you got Feral Scream (silence), slow from your Decrepify , everything ! :) And none of you is squishy.
- Ryze - Great Snare on target Rune Prison , so you can land your Nevermove easily, plus a nice burst of damage.
- Any other Snarer or stunner.

Why i chose those masteries ? Look the reasons :
Good Hands - Less time spent dead, out of 70 seconds in the late game, you stay 63 seconds, helps up a bit.
Perseverence - Always helps some bonus health and mana regen.
Expanded Mind - More Mana, helps Swain a lot.
Greed - More gold.
Meditation - Even more mana regen.
Quickness - Good for some boost of speed.
Blink of an Eye - CD on Flash is shorter :D
Intelligence - CD reduction, always worth it :)
Presence of the Master - Even more cd reduction on Flash & Ignite .
Archmage's Savvy - More Ability Power, combined with Runes, it's great.
Deadliness - Need to put 1 point and if you don't use Smite or Exhaust , thats where you have to put the point.
Sorcery - CD reduction ftw !
Archaic Knowledge - Magic Penetration..you need it !

My best choices are the ones i mentioned.But still, other good choices are :

For Quintessences, you can also go for :
Greater Quintessence of Health - More Health.
Greater Quintessence of Ability Power - More Ability Power in beggining.

For Marks, Magic Penetration ones are the best.

For Seals, you can go with :
Full Greater Seal of Scaling Mana Regeneration or
Full Greater Seal of Replenishment ..depends what you want, flat mana regen or mana regen per level :) Your choice ..

For Glyphs, you can go with :
Full Greater Glyph of Ability Power or
Full Greater Glyph of Scaling Ability Power ...Your choice, full FLAT or FULL per LEVEL.

Im reading comments frequently, so feel free to criticize my build or leave comments about what i should change and what i did wrong. Thanks a lot for reading my guide, I hope it helps !