Re: 1 Pretender and 1 Prophet?
 

They dot tend to give you access to a BIG army. You might have to carefully choose your first water target. Something with just tritons. Once you get one water province you can start building a water army to take the next one.

I usually build a "maybe" army of some water mage and maybe undead or something. Then I watch for some watery mercs to higher. That often signals to me that THIS is the time to strike.