Lets talk about the items I choose and why
Shoes of the MagiIt seems to be agreed that a pair of boots is essential on any God with few exceptions. Some may argue that one is better than another but I believe it comes down to how you play. However if you want to have proper build balance one choice may be better than another. I like to build the
Shoes of the Magi for the pen and damage boost.
Book of Thoth is one of my favorite items for mages. It gives you tons of mana and power and continues to get more powerful as the game goes on.
Gem of Isolation is also a decent choice for your second slot trading quite a bit of power for some really nice utility.
Bancroft's Talon is my choice for the third slot. It gives you a great power boost and a nice life steal boost. Watch as other gods flee in confusion while a squishy Mage goes toe to toe with them.
Pythagorem's Piece is a decent alternative and a good option for multi magic damage teams.
Fourth slot I usually build a defensive piece. If your doing well the other team is gonna start getting upset and right about this time in the match you'll notice a lot more focus coming your way. You could opt to build a power item and it may secure you some kills but I would suggest some D.
Mantle of Discord is always great giving you all the protections you need and a good escape tool.
Spirit Robe is also good with more reliable CC reduction and lower cost in trade for protection.warlock's sash will give you more power and start beefing your health.
For your fourth and fifth slots look at how the enemy is building. If they are wise they will be building some protections to deal with your damage because you'll be laying it on. If not then skip the penetration item and build your final power piece. All the penetration items are effective so pick one. As far as power goes
Rod of Tahuti is a beast and will start blowing fools up.
Soul Reaver is pretty awesome too and I suggest it against people stacking magic protections because it'll just chunk their life away.
I think in Arena, Nu Wa and her clay soldiers would benefit more from health than from protection. The fountain is close by so you can run away to replenish health. Try Warlock Sash as second item instead of Mantle of Discord.