Khergits:
Take your army, lure as many Khergits as possible to follow you (make sure you are as fast, faster, or at least not WAY slower then the Khergit armies). Go into a forest, wait for them, slaughter them.
In general, they will crash into trees, fall off of cliffs, and generally fail at winning the battle. This tactic also works in mountain battles. Assuming you have a half-decent infantry force, they will catch the Khergits crashing into something, and all non-Lancers will be quickly and easily gangraped. The Lancers may get a few kills, but will soon become equally dead as well.
Rhodoks:
In general, the Rhodoks are pretty multi-purpose in terms of terrain. However, the best combo is heavy cavalry, infantry, and archers. If you can, fight them in a flatland battle. Use infantry against spearmen (or horsemen, since Spears generally stink regardless), and do whatever you can to hit the crossbows from behind with cavalry. If you have a Khergit army, circle around as much as possible, and send Lancers when needed to drop their shields.
Usually though, it comes down to who has more troops, and whether they have any sergeants/sharpshooters.
Swadians:
Hold position on a hill, wait. If you can, surround and butcher any cavalry, then charge down and destroy any infantry (due to Swadian infantry not being very strong). When in doubt, swamp them silly.
Nords:
Take your archers, use them first. When the Nords get close, charge. Ideally, hit them while they are crossing a river so as to surround and slaughter them.
Vaegirs:
Get your cavalry to follow you and your infantry/archers to hold position near a defensible position (either a river or a hill).
If possible, swamp any cavalry charges. If not, have your infantry keep the Vaegir infantry distracted. Charge their archers with your cavalry. Kill them all, then surround and destroy any Vaegir survivors.
