Playing Hard battles is, well, hard! The mod was balanced for Medium difficulty, and Hard battles increase the difficulty of small factions by a lot.
As Pontos I recommend to take Mazaka first: if you're lucky you can do it on T1 with a spy and Ktistes Mithradates leading troops. There are different schools of thought about the follow-up: the cheesiest way to play is to kick the Seleukids out of Anatolia entirely, meaning that you should conquer Ipsos and Sardis as soon as possible, and then take the rebel cities when you're well set-up. I prefer to go for the hardest strategy, which is to take Sinope and Ankyra, possibly baiting the defenders to sally thanks to my small sieging army and beating them in the open.
With Amaseia, Mazaka and Sinope I was on positive balance; with Ankyra I had a good amount of spare money to invest in army, so I could take Ipsos and Halikarnassos. From this point on the campaign gets easier, so I usually attack the Ptolemaioi in order to divert their attention from the Seleukids (who get beaten pretty often in my campaigns). Also, keep in mind that taking Trapezous usually pisses off the Hayasdan, so take it into account.
Talking about Hayasdan… That's another hard campaign for sure. You need to conquer the settlements to the north and the east (don't remember their names, unfortunately) as soon as possible, otherwise the Sauromatae will come for them. Then go south and kick the Seleukids out of their northernmost city. Don't bother with Trapezous and Mazaka too soon, otherwise you piss off Pontos.
If you have questions, feel free to ask.
Good luck for your future campaigns!