i like to build swain tanky, with some ap and some armor, like an off tank but not that tank.
start off with sapphire crystal (blue one)
-first trip catalyst, if ur fed get boots and always, pots and wards
-second base is ages + wards + (upgrade boots) if fed and if u can, since at that time u'll be heading drake a lot of times, if u can get ages before the 10 min (usual time drake is gonna get slaughtered)
-finish up boots(if massive CC and Ap, get mercury, if not sorcerers work), get spirit visage, after that rylai, then 1 negatron, and one frozen heart). transform negratron into banshee's or nature.
of course this may vary, only thing i know is that ages and rylai's are a must, getting rabaddon also helps if ur team doesnt has any AP. mostly on pubs nobody wants to tank so building it tanky helps a lot, but on pubs, pre-made are different