I don't play Diana as a tank, if you're looking to play this champion as a tank you are wasting a slot on your team. while she does have a shield, its used primarily for dive survival. This guide is simple and will give you a list of the items and mastery's I use to win with Di.

The biggest ticket item on my Di build is the Mejai's Soulstealer. It's cheap, but when you max it out (and I almost always do) it gives 160 AP (208 AP with a Rabadon's).