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Originally Posted by Hiclic
Considering you're low level, that's an nice build. As you progress you should naturally learn how to control mana even with harass. Masteries/runes will help out later. Mana won't be as much an issue early game as it currently is and you can skip your tear for more direct builds.
Lately, I've been finding an early AP-tanky+blue pot setup works wonders. At my optimum so far, I duo queue in ranked. So I need to make sure I can survive if my teammates don't all show up and perform--i.e. NOT meta game. For this reason, Boots, 2x Dorans Ring and a Rylai's make a very tanky, damaging and otherwise useful core. I usually start with BoS+2x HP Pots + 1x Mana Pot. That way I can harass most mids ad nauseum. If you can't tell who your mid opponent is going to be, you cannot go a build like Saph Crystal. It leaves you open to harass from opponents with pots and even doran's. If you see your opponents catching on, adapt! Simple as that. Never be afraid of going b instead of trading your life and you should do very well throughout your career.
Rod of Ages is an item taken if you expect a very long game. Otherwise it's usually a waste. I consider the AP tank triumvirate to be Rylais (Slow+HP), Zhonyas (Armor+activate), and Abyssal Scepter (MR+MR debuff). RoA needs to be bought early, but pays off late. Most AP users get powerful early, so you hamper your class given power in a crucial game-swing phase. I wouldn't recommend it on an AP DPS like Cass. She needs enough tank/positioning/damage to get her job done, RoA is just too slanted towards tank that you'll have trouble getting the rest of your job finished.
I tried 3 games yesterday following this guide's suggestions and going for Archangel's Staff as 1st main equip and it works very well followed by a Giant's Belt and right after I go for Rabadon's Deathcap. If I still have money after that, I go for Rylai's Crystal Scepter since I've bought the Belt already. The HP bonus the belt gives is really good and even if I don't have time and/or money to finish the scepter, I still have the option to make it. =)