So far as Atropatene I've only managed sucess with a friend in multiplayer being Maurya and helping me occasionally. Otherwise I find the Seleucids and their billion allies swarm me, even if I optimize my armies to be able to defeat theirs in open battle, there are just too many of theirs.
Yeah like what
@IsaacCAT sad;
You don't win the war by fighting battles. You really have to play on slower speed for the first 1 or 2 wars.
1) WAIT.. If the seleucids don't get sucked into a LOA war with Antigonids, I'd advise starting over.
2)Integrate cadusian pops, this can take a few years so let the Seleucids get sucked into war far in the west.
3) Ally Armenia, improve relations and send gifts. If they 'plan your demise' then ally Albania too even if this means going over diplo slots. This should deter Armenia from declaring war.
4)At about 454-455 when the Seleucids are still fighting the Antigonids raise your levies.
5) Choose your wargoal very carefully. Make sure its easy to siege down quickly and easy to defend. The 25 warscore is important. Call in Armenia.
6) Take time managing your levies, I divide them into 3-4 mini stack of cavalry units that will sprint far into seleucid lands and SACK cities for as much gold as possible (King commanding all of them). Speed is important here so have cavalry lined up on the border like the start of a horserace here. The seleucids won't match you for speed. Sack as far as Persepolis and into the Persian Gulf all along the coast so that as much land is occupied as possible. If you have only cavalry in your small raiding units you'll be surprised how much work they can do. AVOID battles.
7) Hold back ALL your Infantry and immediately siege down the wargoal and as many forts as possible, Hopefully Armenia should have at least 15,000-20,000 levies that can take care of one seleucid army returning from the West buying you some time. Avoid Bactrian stacks as much as possible and try to lure them with your cav deep into Persia to recover cities that will keep them away from the wargoal. Send some cavalry near Bactria to scout them out and lure them away from your land.
8) Your warchest should be huge from sacking. Hire mercenaries, as large as you can and with the best commander. The seleucids are going to start sending mercenaries soon as they can't reach you fast enough with their armies and the Bactrians are busy chasing your small quick cav units around.
9) You should have the wargoal and much of the Eastern lands occupied and now is the time for some battles so choose wisely. defend the wargoal and use mercenaries to fight small stacks where possible.
10) If done well you should have 50ish warscore by now, take 2-3 provinces as if you leave the war drag on you'll eventually find its the seleucids who have more resources. You should be able to form Media.
11) For the second war use the same strategy, but your starting position will be even stronger. Rinse and repeat.