10 Spell Arena Build

136 HP
91% Cast Speed (105% with haste)
40% Magic Power Bonus (extra with Mana Sphere out)
30% Magic Pen w/ religion + Mana Sphere

Gear Options

Chest: You can swap this for a Light Aketon, Dark Matter Tunic, Oracles, Pastoral - many options to swap here
Primary: On top of Mana Sphere carry a Tome of Sheol as well. If you notice you need some fast casting and move speed swap to Tome mid game. But if you have more space and can spare a little longer cast time for the damage and true damage use Sphere. NOTE that BOTH have 10% pen so you will hit 30% mag pen regardless.