Going for the tear as your first main item (besides boots) is important so you can start building stacks for the archangel's staff later on. His abilities scale largely off of mana which is why you can focus more on that than ap. Also the rod of ages will give the most mana available plus health AND ap as well. As the game goes on you will only get stronger being able to pick off enemies at your will.

Mainly you want to get overload up first since A. it has the shortest cooldown and B. it does the most damage with this build. Overload will do lots of damage based on your max mana. It may not seem like much but when combined with his passive you can use that ability 3 times in the period of a few seconds. I always open with overload then combo with rune prison, back to overload, spell flux, and if they are still alive, another overload should do the trick. Of course in a team fight use desperate power so your abilities are causing aoe damage and giving you health back.