I main Diana mid and I would greatly recommend her as a Kassadin counter. I also started playing only long ranged mages at mid, Lux, Lissandra, Syndra. Diana is a great champ to pick up if you want to bridge your gap from a long range mage mid, to a melee assassin mid. Early game you'll go about even with Kass, Diana has a weak laning phase so you'll want to play passively, while poking him when he doesn't have a shield up. I typically start crystalline flask and three health pots for early sustainability against his poke. However once you hit six you can completely out trade him, as Diana will generally have more burst potential. Also thanks to her ultimate refreshes it is also fairly easy to chase down a Kassadin. Also on another note, Diana counters lots of other meta mids. I typically play her against Zed, Akali, Kat, Yasuo, and if you're playing against an ap mage, then the lane is even easier. The only lane I really struggle with as Diana is against a Hiemerdinger, but even that isn't too bad you play smart. Also if you only play mid Diana is great for learning to top/jungle. Anyway sorry for rambling; Take Diana into Kassadan>Make Kassadin cry.