New items arent that great for AP mids, only Liandry's Torment and Twin Shadows make an impact, one for dealing with tanks/bruisers and the other one to detect and prevent ganks. Im building most of my AP champs like I used to before.
Swain is stupid strong now with Liandrys (not that he wasn't before).
I've been going RoA - Liandrys - Rylais as core, boots when you feel comfortable fitting them in.
Start with lots of sustain (starting boots is kind of a waste now), rush that catalyst, and just farm and push your lane opponent around. Once you get Liandrys if your team hasn't been crushed in laning just start forcing teamfights everywhere and it's almost impossible for you to lose a 5v5.