Swain is an amazing champion and one of the tankiest mages this build is a more tanky sustain build. Swain also is not a bursty champ he can whittle down a champ little by little then swoop in for the kill

Swain has very poor farmability I know I made that word up. He cant take down waves and waves of minions since he has more single target spells and his W has a high cooldown. In crow form though he can take out a lot of minions it just takes a lot of mana.

swains spells have a wide variety of combo sequences and ways to trap your foes.
Q slow effect allows you to easily set a W to catch them or just slow them enough for you to catch up with them in crow form.
W has slight burst potential but makes an awesome trap but has some weaknesses when cast. In a bush it does NOT reveal the Area of cast so remember that. Chaining W with a Q or E is helpful and gives you the chance to catch your enemy off guard it has pretty wide cast radius and can be cast about halfway outside its cast radius for a little extra distance.
Swains E I level first at max level it makes all other spells 20% more effective to the champ afflicted with it. !INCLUDING IGNITE! also chaining his E with deathfire grasp can make your spells 40% more effective to your foes.

These items that I have selected, build as you will. With these you will be fairly tanky and have a TON of mana, and can stay in crow form for a prolonged amount of time and cast a flurry of spells to cut your enemies down.
ROA gives you immediate sustain and mana from an early game. Building catalyst or sapphire crystal on first recall is recommended to get this item early.
Morello will give you cooldown, and will prevent healing champs from healing through your spells due to grievous wound effect.
Archangels and Iceborn and ROA stack to give you about an extra 2200 mana and about 80 ap due to archangels effect. Archangels evolves very quickly on swain due to his ult every second counts as a mana expenditure so you gain that extra ap and mana fast.
Lastly is deathfire if you want you can switch this out with a zhonyas for more tankyness or a rabadons for more ap if you wish, but the active effect of deathfire combined with swains E makes all spells INCLUDING IGNITE! 40% more effective so it almost doubles his damage output