The Daevanum Empire has many times tried to put an end to the Reuvic raids in their borderlands. Despite being by no means a sick empire (most crises that historians point as the cause of their failure actually were results of the defeats), the empire has consistently being defeated in all campaings against the reuven people.
And worst of all, those are generally not even that close fought! In all but three of these wars, the result was massive raids against the empire's heartland and/or prosperous provinces. Somehow, these tribes have more often then not beaten well-provisioned, trained and experient professional soldiers, as well as the well-educated commanders that lead them.
These tribes do have a few things in their favor:
Their society, while tribal and politically divided, is extremally cohesive. The moment that a outsiders threatens even a minor tribe, suddenly the tribes gather to elect a war-king and start acting like a nation-state.
Their terrain do put a strain on logistics. So they have it easier while defending their own lands, but it does not explain how they keep hitting back at the empire in their own territory
They have technological parity with the empire, and their economy its not that bad. Between self production, trade and raids, they are about as well equipped as the imperial soldiers. Both the empire and the tribes have about the technology of the roman and parthian empires.
They are primarily sedentary herders, but with a "nomadic tradition", they have pemanent settlement, but will temporarily move if needed. I think this might help as you can move your flocks to safe/hidden places, while grains in the field can be burned by enemy troops.
Their culture is quite focus on war, if something is compatible with their lifestyle and would help in a war, they will try to do it. This does not apply to political centralization and greater intertribe cooperation when at peace.
They usually can fill a bigger cavalry than the empire, but no steppe nomad levels of cavalry. They have horse archers (mostly from their tribal nobility), which the empire lacks.
However, none of this gives the discipline needed to beat an army. Most engagements between tribal armies and professianal armies (that I am aware of) are won by the professional due to the tribes panicking and rooting while the professional could keep it together for longer, such as reserves engaging rather than fleeing when a flack its about to break, or breaking formation to persue the enemy and being crushed by the reorganized enemy troops.
The problem is that this less disciplined way of fighting generally is what makes sense in tribal warfare, as they usually don't use forces large enough to make individual skill less valuable than discipline and army cohesion. On top of that, tribes usually lack the economy to wage long periods of warfare or keep armies well supplied, meaning that they generally will rush to the engagement while they still have the resources to do so, which gives the empire a huge strategical advantage.
My first though was: "Well, nomadic tribes conquer empires all the time, it can be that hard". But the nomadic people advantages that my tribes lack, namely being nomadic make so that there is no important settlement for your enemies to seize and force a surrender (and also obscene amounts of horses). Besides, the occasional tribes conquer big empire in history usually coincide with the empire already colapsing due to other factors, while the Daevan-Reuvic Wars always start with the empire on offensive, thus bound to not be near colapse every time they try.
How can the tribal people overcome these problems? Can they find a way of fostering discipline and unit cohesion in their warriors or better feeding their armies? Or they can only try to go around this problems? What strategies or tatics would give them a edge over a better organized force?