In fact they haven't bothered me.
The WRE have received considerable damage from the Quadi and the ERE has been also annihilated by the Sassanids. So I stayed alone in the Middle East, with a badly damaged stack of legio palatinae units and the Sassanids are sending 3 full stacks of cataphracts against me each turn. I have found it impossible to cope up with them so I just stopped playing this campaign.
I'm not a bad player, if you give me the WRE I can do miracles but against the Sassanid Empire
I'm really unable to defeat them.