This will not work well against False Gods and it's not amazing for the Arena, but it can get the job done if you want it to I suppose. As far as upgrading though, it's mostly preference. I've always preferred upgrading my 'economy' sooner than later though, so I'd suggest something like this:
1. 6x Immolation -> Cremation
2. 4x Phoenix -> Minor Phoenix
3. 6x Pillar -> Tower
4. 5x Golem -> Destroyer
5. 1x Hammer -> Gavel
6. 2x Deflag -> Explosion
7. 4x Photon -> Ray of Light
8. 2x Seraph -> Seraph (I would probably use 2x Phoenix until I upgraded the Photons as a minimum before replacing them with Seraphim, preferably upgraded ones)