When can I first use my God for fighting?

[tratorix]

Titans can't expand early on by themselves. For Arco I'd recommend either a Wyrm with no magic paths or a Great White Bull. Make sure they have at least a dominion score of 9, which will give them the awe ability, forcing any unit that attacks them to make a morale check. Watch out for cavalry, lances do a lot of damage on that first strike.

Edit: Baal'z thug guide might also be useful to you. While it doesn't necessarily cover awake pretenders, it might help you get a better understanding of crowd control and what can expand early on.