The Rhodoks are an unusual faction; whereas the Khergits are cavalry-focussed the Rhodoks have only infantry and crossbowmen. Their spearmen have a combat advantage against cavalry but are only moderately useful against other infantry.
Combat strategies
Against infantry
The individual Rhodok infantrymen is a poor fighter, a large band of them in tight formation is a different story all together. Though it may be preferable to have your crossbowman bombard the enemy to kingdom come, if you need to engage in melee, have your Rhodok infantry advance as a large mass where the reach of their spears can cover each others backs and deliver pointy death to their enemies.
Against cavalry
If you have a mixed array of rhodok troops, then the best thing to do is to order your infantry ahead of your crossbowman, on a hill this works much better, and as the cavalry approach, they will be knocked down by the heavy fire of crossbow bolts. Meanwhile, your rhodok infantry will have a very good chance in taking down the infantry without a problem, unless of course you are fighting a bunch of hired blades, with only a handful of spearmen.
Against archers
Archers are the bane of infantry and should be dealt with quickly. If engaging archers on flat ground have your crossbowman spread out and charge in. They will advance from the front and provide covering fire whilst drawing the barrage from the enemy archers.
Have your spearmen and sergeants pack themselves in a tight formation and move from the sides to outflank the archers either from the sides or the rear.
If engaging archers on uneven and mountainous terrain, the best thing to do is to lure them to you. Bowmen may fire faster but this also means they run out of ammunition quicker. Have your troops hide in a sheltered location and ride out to distract and draw their fire. Once their ammunition is used up they should make easy prey for your men.
Against ranged horsemen
If your army is heavy with crossbowmen, mainly trained crossbowmen and sharpshooters your best bet is to get to high ground, ranged horsemen are not quite as accurate as your ranged units and you therefore will have a good kill to death ratio, standing on top of hills also make your soldiers naturally harder to hit. Steep hills also do very well in limiting the enemy's movement speed which against your slow infantry is their biggest advantage.
If your forces consist mainly of melee infantry units your best bet is to try to station them on high ground or in a river, since most of your infantry, for some reason sergeants seem to get twohanded weapons quite often, will have access to the board shield. This makes them almost impervious to frontal archery assaults, your best bet is to wait until the enemy run out of arrows after which the poor AI will attack your shield wall one after another making for easy kills.