The Khergits have some of the most distinctive advantages and disadvantages among the Calradian nations.
Using the Khergit Khanate[]
Campaign[]
Not only does the Khanate start out with four towns, but they have the most mobile armies in all of Calradia. With just a little luck, you can easily kill, pillage, and burn the countryside of every nation in the game, as well as easily win field battles (even against Rhodoks, if you put a little planning into it). In sieges, all but one of their units having missile weapons pays off in attacking and defending castles, though their light armor makes them weak in melee. The best advice for the aspiring Khergit lord would be either to go and get troops from another faction to assist in sieges or completely overwhelm your opponents with your firepower.
Additionally, foreign soldiers are also necessary for a successful Khergit army because of the unique disadvantages from the exceptionally detrimental tactical rigidity caused by lacking any form of infantry. Though the most obvious need for infantry is besieging and defending fortified fiefs, there are also some critical roles in the field that can only filled by infantry. Against a solid shield wall with protected flanks and/or on a battlefield that's particularly uneven or wet, a pure Khergit army will inevitably suffer substantial losses, if not be defeated entirely. Dismounted Khergit soldiers do offset some of this problem, but on foot, Veteran Horse Archers and especially lancers are easily matched or bettered by almost any other nation's foot soldiers.
Although you will probably be best off using Rhodok troops to defend your towns/castles. For field battles, pick mobile, hard hitting infantry such as Huscarls, Rhodok Sergeants and Vaegir Guards to support your cavalry. Heavy cavalry from other factions can also be used to support your khergit warband if lancers don't do the trick.
Battlefield[]
You may use any Hotkey for the units, and may use any units from any faction. However, certain bandits and the Khergits are the only ones with ranged cavalry. The "Lancer" unit may be any cavalry type.
Khergits have the simplest tactics in the game. Against most troops, they can be trusted to do fairly well simply by charging at the enemy. If the enemy has higher level infantry, particularly Rhodok infantry, then it is prudent to tell your cavalry to follow you, while you ride in circles around the enemy. This allows you to soften up the enemy slowly with your archers. It is also a good idea to lead a lancer charge through the enemy every once in a while to break up their line of defense.
Alternatively, a slightly different and somewhat more sophisticated tactic can be used to devastating effect with your skilled Khergit horse archers: the Cantabrian circle, which simply involves leading your cavalry around in a moving, circular path at a bit of distance from the enemy lines. Though it does require more open terrain than simple circling, and the single file formation traditionally used historically is difficult to maneuver your troops into for obvious reasons, it still confers a couple of powerful advantages. The fact that the people closest to the enemy ranks and firing are effectively swapped out for other people as the circle spins means that there is a constant, withering rain of arrows upon your enemy as some of your horsemen are shooting while the others are reloading and ready their next shot. Plus, the constant shifting movement and the simple fact that only less than half of your archers are ever exposed to the enemy at any one time makes it incredibly more difficult for your enemy's ranged troops to land any shots on you or your soldiers. This tactic and other similar ones such as the Parthian shot (simply constantly keeping ahead of the enemy and shooting arrows backwards until they're dead) were historically used to great effect against the infantry ranks of the Roman Empire by the eponymous Cantabri tribe and horse archers from various cultures of the Eurasian steppes.
Another historically based tactic is to have three cavalry types under different group names (see image). Have your strongest melee cavalry (preferably Lancers) charge into the enemy. The other two sides will be Horse Archers or Veteran Horse Archers. Order one side to pass the enemy via the "Advance ten paces" command (you may want to lead these cavalry archers away with the "Follow me" command so these archer don't collide with the enemy line). Have the stronger cavalry archers to follow you and flank the enemy lines.
After a battle has gone on for some time, it may be a good idea to order all your troops to "Mount Horses" as some riders may have been killed or dismounted. This has the added benefit of putting surviving Tribesmen onto horseback so they have an easier time staying alive for the rest of the battle.
Prisoners[]
Khergit units are almost always mounted. This means that they have horses and horses can deal charging damage which knocks soldiers unconscious.
If you're looking to take some prisoners against an army comprised entirely of infantry (mostly Nords and Rhodoks), tell your Khergit army to use their blunt weapons. The Lancers will obediently take out clubs and start bashing away. The Horse Archers won't. Tell your half blunt-weapon army to follow you, and charge towards the infantry-based army. Once you have clubbed your way through the sea of infantry while your horsemen behind deal some charging damage, turn around, and repeat. Do this a couple of times, and hopefully you can take the whole army prisoner. Make sure the army doesn't have any cavalry, as they can quickly spoil this prisoner-taking tactic.
Defeating the Khergit Khanate[]
Fighting Khergits is challenging, even with a tactical approach. Seeing how Khergits completely swarm their enemy with cavalry charges at the beginning of a battle, a commander has less time to think of tactics before the enemy approaches. This can also create a problem in moving infantry or archers into position. To create more time, a commander can set up a wall of infantry and/or cavalry. Archers are not recommended in a blockade, especially not Vaegir Archers or Swadian Crossbowmen. Rhodok Crossbowmen, Sarranid Archers, and Nord Archers can generally stand their ground, but usually fall to Khergits due to their lighter armor and weapon stats. Terrain advantages can also slow down Khergits (especially mountains and streams). When terrain advantages are exploited and either Rhodok or Nord infantry are deployed in a shield wall formation (ideally also with supporting missile troops behind and/or cavalry cover for the flanks), Khergit cavalry charges can be easily stopped in their tracks and wasted by your spears or axes with minimal losses.
It is also notable to set all your infantry (and possibly cavalry) troops in holding. Then have them stand extremely close, effectively creating a slaughterhouse. When the horses charge into your dense cover of pointy spears and swords, their horses are turned to pulp, and their lancers will either switch to clubs or keep their lances. Which will not do much damage. There is also the chance horses will not die. Allowing them to be living pin cushion for the archers circling you. After you have destroyed their main forces. Send your men out in charge to have them hunt down the rest of the remaining troops.
If you are riding a tough horse such as a charger and are wearing decent armor, another potential tactic at the start of a battle is to ride in front of the path of their charge and turn your horse sideways, using it as a barricade. This will cause the front line of horsemen to suddenly stop which will ripple back, canceling all of the momentum of the charge making it less likely for your infantry to get skewered by a lance, Be on the lookout for lancers on the first line of horseman to avoid getting hit yourself. This will also enable your archers to get a few pop shots in.