Well, I noticed the AI ignores parthia for the most part, and only focuses on rebels. parthia has a huge army, strong and good vassals. Rome starts with a bigger army, and amazing trade. So, instead of dealing with the rebels, I mostly ignored them with my main army and sent almost everyone to the east to meet parthia on the battlefield.
Rebels as predicted very quickly started rampaging through the empire, and even some in the UK. I bought tons of mercenaries all over, about 40 mercenaries in total (my main army having a little over 70 troops out fighting parthia) and dealt with the rebels coming up. But they weren't too big a deal, unlike parthia.
first huge battle against parthia? my 70 or so troops against parthia's horde of 80., including its vassal's armies. well, lost that one...was trying to stop them going but it was already to late to change direction and i was fighting a river battle. there is some fog of war early on, so couldn't see their army coming.
my troops replenish themselves and I take armenia (which was the war goal) and use my army to pick off their troops they broke off into various areas. it turns out, parthia should have stuck together ;p the battles after that were a lot easier. Except the rebels...
i look at rome and almost the entire italy is controlled by rebels and more rebels popping up everywhere. i send out my mercenaries to deal with them.
The next many years up to near the end of year 66, is massive battles all over the place. So many romans, parthians, armenians dead, along with the other two vassals of parthia.
In total there must have been over 80 thousand dead romans, and over 200 thousand dead parthians...a bloody war for both, but parthia definitely got beat pretty bad.
Now, for the next many decades in-game...no expanding AT ALL, keep borders defended, make everyone the same religion and start bringing down unrest. oh, and fix my economy lol...
its so different playing romans at year 58. so hard at beginning dealing with parthia and mass rebellions, but then after that you can't expand for so long, cause so many provinces need to be converted. and only one missionary ;p
oh and I did make armenia and two other vassals under control by parthia, my vassals...thats a pretty big defeat for parthia lol. I didn't take any territory, just vassals