Let me see if I get the "logic" of this build. You are trying to spam volley and slow during all game? Your main dmg output comes from manamune? You have like 4 Mana items and end up with more than 2k Mana.

At mid game you will hit no more than 300 dmg to normal champions, not to mention that you will make no real damage at all to tanks.

Also you have tons of Mana regen runes that just gives you MORE Mana regen. Try getting movement speed Quints wich will work great with your slow.

If you believe that spamming slow is the main goal of this build, I would strongly recommend to get Nashor's Tooth instead of Trinity Edge (CD reduction, Atk speed and mana regen... everything you need)