You look bigger and better than them. If they have lots of phalanxes my suggestion is to build lots of legionary heavy infantry supported by at least three units of Prodromoi (Thraikoi are the best) or some good flanking cavalry. Focus your power on routing a few phalanxes and their auxiliaries first, bypass the rest and do it methodically during battle until they are all dead. Don't engage them unless you plan on taking them soon, since the toll will be heavy.

Drag the Sauromatae into your war: HA's are certainly their achilles heel, besides it will cause them further distraction while you plan on beating them seriously. Pay them as much gold as they demand since your economy should be robust by now.