First, you need to have a really, really good control of Swain because
- Unlike many mages, two of his main skills are single target Dot spells. You have to actually approach to enemies to attack and take some risk.
- His ult uses lots of mana
- His Nevermove(W) takes 1.5 sec to root enemies so you need a good aim.
- You have no escape route except W, which is quite easy to avoid. That means you are very susceptible to ganks.
Search Swain Bird is the word guide in mobafire. That's probably the best guide.
Don't buy any mana regen item. Swain's ult mana expenditure increases 5 per sec, with base 25 sec. You can't use your ult for a very long time and athene, morellonomicon, and even blue buff can't catch up with that.
Build RoA first. It gives you mana, health and extra AP in each 1 minute, increasing Swain's tankiness. If you are doing REALLY bad, buy three Doran's ring as a "fake RoA". This is why many conventional AP champs can easily push him in lances but can't kill him fast without good gankings. They eventually run out of mana and keep AAing or go back.
I usually pick Tear and other normal AP item like Blasting wand, then sell Tear when I need other items and earned about 350 mana. Not many people know this, but the extra man will come back once you re-buy Tear.
Since RoA is enough for most of games, Seraphs embrace may not be the best one, but still a decent choice when you are fighting agaisnt super tanky champs By the late game, I usually have rabadon, Void staff, Spirit Visage(for more heal), Sorcere's shoes, RoA, and Abysal Scepter.