You just have to be agressive from the start. I avoided attacking/ beseiging rebel cities and focussed on absorbing the Seleucid cities closest to me. Then I created a full stack, imagined myself a Mongol khan and went about filling my coffers with gold as I went through and raised almost every Seleucid city to my south east. As I did this I upgraded my mines and beseiged and absorbed the remaining two rebel cities in the Caucasus. If you can get this far you're in a reasonably strong position. Happy hunting.