A few other points on Arco.
Arco has great access to multiple paths of magic. As such they can search and forge alot of different items. Most of the spells, summons and items with a path 4 requirement or lower you will be able to use. If your pretender has a physically strong form you might want to put some of the better items on him and send him out to crush armies with only a light vanguard.
Hoplites while slow are very strong. They can hold back and fight even very strong odds because they have shields and very long weapons. They are best used at a front building in a castle. They are resource intensive, but well worth the costs. They also have very decent morale.
Priestesses can heal Afflictions as well as cast Protection. Don't be afraid to have these around healing whoever happens to be in the province they are. The magic phase is before the movement phase so someone can get healed and move out in the same turn.
Arco's scrying domain lets them know quite a bit about what is coming and how to react. Don't be afraid to use your instant access spell of Astral Window as soon as you find an opponents capital in order to look know what's going on in each of their production centers. This will also let you know who and when people are going to 'change their mind' and attack
One Independant you might want to look for/keep an eye on/build a castle on is a decent archer. Woodsmen and Hoburg Xbows are both very good for this.
Watch out for using chariots in the same fashion you use Elephants. They rout and will trample through your own lines unless they have a strong backbone. They are also size 4, so if you do put them on a flank, and your opponent uses cavalry, you will be able to trample them.
I'd shy away from building any mounted with Arco, as none of them have lances.
Slingers make excellent patrollers if you don't happen to have any air gems for a Call of the Wind.