Sycathians are really a pain, there are rich, and bulding stacks of Sycathian Noble Horse Archer that are very difficult to beat.
In my Parthian campaing, I tried to hold them with forts, and fight them on birdges. While holdind them with forts, I invaded asia minor and quickly build a navy in Black sea, and blocaded all Sycathian forts that made them slower a bit.
But I am also curious about which troops are effective aganist horde of Sycathian Noble Horse Arhers.
I think Aor Bactrian archers-for their secondary spear weapon, Median Cavalry- for their armour piercing axes and powerfull missles may work but I find these units very expensive and hard to recruit in good numbers though.