This is 3 quite standard Swain builds, that focuses more on tankiness then straight stand and shoot mage.
As everyone know, he benefits great of AP, HP and Mana. But I have left out Archangels staff of all the builds. Except all the mana and AP it can offer, it offer nothing else. there are greater things to gain at other items. If u play with a good team ( and there is not aniva etc) they always let Swain get the blue buff.
I have gone with The Defense Mastery tree here, as u want to be able to choose to be in the thick of it. Yes the utliity tree is great but his passive should cover some of it (and u get the blue buff).
I even chose some dodge runes, instead of Mana regen. If u are good at mana handling with him, Dodge runes can save youre neck.

BUILD 1
This should be the most standard build. Some people scream Mejai's Soulstealer, and I like it on every
mage, but actually on Swain, he doesnt need it as much as other champs, especially as he needs other stat quite much.
The items in this build is nothing strange. It is the build of the 3 of them that dishes
out most damages as the same time u can be somewhat protected aganst magic damage. AND
Will of the Ancients and Abyssal Mask helps the team with some good auras that synergizes well with Swain also. Will of the Ancients is some of the last items, because sometimes, u dont need the Spellvamp, and/or the other in team is AD or don´t dish out lot of Magic damage. In that case choose Deathfire Grasp or maybe Zhonya's Hourglass instead, as they offer more then.

BUILD 2
This is an anti AD build. As it it doens help out your team as much, the extra armour from Zhonya's Hourglass and HP from Rod of Ages (+mana), makes u less viable for focus,
and can survive to get safetly to tower if u get ganked. The second Rod of Ages can be placed after the first if u are a quick farmer. I got the Rylai's scepter before
Rabadon's Deathcap because AD (melee and Range) can be quite quick early on.

BUILD 3
This is the antitank build. Should be used when opponents have 2 tanks and maybe 1 high hp champ or more. I´ve put the anti tank things last because most often tanks get as most MR in the last and Void Staff isnt as viable yet. MP Runes, masteries and Shoes should be enough for some time. If the tanks isnt stacking MR for some reason, pick something else.

2 VS 1
Quite obvious but in case u need to defend a 2 vs 1, Skill-up Nevermore first, so u can clear waves quickly. In some cases also when u go 2vs2 and feel that the other team got the upper hand on your lane it can be good to skill up Nevermore first also.