I downvoted for several reasons:
1- What a huge block of text!! not even a link for items!!
2- So short, need to explain why that order of powers, runes, masteries...
3- Pro great burst damage? are u crazy? the most powerfull part of swain is his sustained damage... his ult, 2 dots... sustained, not instant damage...
4- Hextech gunblade??? why??? why not woa?? whats the benefit of taking so little ad and life steal if u practically hit only whit ap?? that not make sense.
5- Why mejai soulstealer? its a high risk item, i think u could go better directli to rabbadon, or Abbysal, for mr reduction (works good whit your short range ult) plus mr for u and ap, zhonya for an invulnerable vampire who can recharge his life bar in 2 secs whit nothing to counter that, or rylai, who adds more cc whit every crow instead an item who only can give u flat ap and potentialy 15% cdr, but only if u not die, and u are swain, u need to stay close cause your ult is a short ranged aura...
6- Why archagel staff as last item? u need time to get his stacks, if not u are only trowing away your money. Or u get that at the begin, or forget that item in the whole game.

So bad man, i cant think u are thinking right your build, even if it works its not the ideal build...

P.D. Sorry for my english, im bad whit this languaje but i really want to said this and try to help you

I downvoted for several reasons:
1- What a huge block of text!! not even a link for items!!
2- So short, need to explain why that order of powers, runes, masteries...
3- Pro great burst damage? are u crazy? the most powerfull part of swain is his sustained damage... his ult, 2 dots... sustained, not instant damage...
4- Hextech gunblade??? why??? why not woa?? whats the benefit of taking so little ad and life steal if u practically hit only whit ap?? that not make sense.
5- Why mejai soulstealer? its a high risk item, i think u could go better directli to rabbadon, or Abbysal, for mr reduction (works good whit your short range ult) plus mr for u and ap, zhonya for an invulnerable vampire who can recharge his life bar in 2 secs whit nothing to counter that, or rylai, who adds more cc whit every crow instead an item who only can give u flat ap and potentialy 15% cdr, but only if u not die, and u are swain, u need to stay close cause your ult is a short ranged aura...
6- Why archagel staff as last item? u need time to get his stacks, if not u are only trowing away your money. Or u get that at the begin, or forget that item in the whole game.

So bad man, i cant think u are thinking right your build, even if it works its not the ideal build...

P.D. Sorry for my english, im bad whit this languaje but i really want to said this and try to help you

jj but you need really work on it, some pictures, more description of the items, of skills, maybe other item choises.

No, they my mates. i asked them to have a look

pretty wall of text....better explain and the 2x likes seems be also your work when im see the date of creation of the accs.

nice guide