first get the dorans shield cause you will need that extra health and health regen. what you want to do is harass like crazy with your shard and if you run out of mana use your clarity. once you get your ultimate you can just unload on a champion and get a easy kill. then go back and either get merc treds or ninja tabis depending on the team.

during mid game you want to make sure you have your sunfire cape and force or nature so you have that extra health regen and so you have that extra damage on you enemy when you ulti them or just tank the damage.

now for late game just remember that your ultimate is very important. make sure to use it on as may people at once or go by who you want to focus first. if you get the full built you should be able to just ulti in and keep hammering down on them.