Since i am a lazy guy,i won't write the same things as in the other guides.You all know kassadin and what he can do or not do.The only thing you maybe interested for reading my guide is why i don't add Archangel's Staff in my build.So,Kassadin is a burst damage meelee ap champion.As a meelee ap champion one of the necessary items for something more than burst but also sustained damage is lich bane.Back to our problem now,why not Archangel's.Since you have Rod of Ages you have the necessary hp and mana.Yes,i know Kassadin can be a mana consuming beast when it comes to his R.But here is the problem,it's all a matter of gameplay.With my build you don't have to gather stacks of his R to do extreme damage because you already do tons of damage!Ok yes,when you are chasing somebody you may go out of mana but only if you chase him throughout the map but i think that's logical since the mana you are going to need for 10xR is 5500 which even Archangel's doesn't give.Anyway,that's my own playstyle and i won't provoke you to use it.It's your choice.For beginners the only thing i will say,follow the combo! (R-Q-E-W) or (R-Q-W if you don't have E stacks). Have fun dominating the summoner's rift and any known world :D!