With the introduction of many new and unique aspects of Cossacks: European Wars, guerilla tactics are much more viable that in their RTS predecessors. There are even a wider range of tactics to be employed, each with a highly useful and specialistic manner.
Parthian Tactics(Hit and run): This tactic is actually quite simple. A player simply needs to get a small number of ranged cavalrymen(such as dragoons) and order them to attack the slower troops of the enemy. When the enemy gets too close to the attackers, order them to retreat to a safer distance. This will slowly wear down the enemy, making him/her suceptable to a full scale frontal assault.
Parthian Tactics Antidote: The most useful way to null the power of a hit and run strategy is to employ light cavalry units to charge and kill the slower ranged cavalry. If possible, it would be useful to have a small ranged infantry support, because light cavalry themselves are rather weak. It would also further weaken the attack if you were to have a couple of quick heavy cavalry units(such as cuirassiers or hussars) to help your light cavalry, though it is not absolutely vital, you can do without them.
Assasinations(kill drummers and officers): Assasinating is essentially using ranged cavalry units(such as dragoons) to pick off and kill the drummers and officers of a formation. In order to successfully kill officers and drummers, one generally must use this technique. Group a very small amount of ranged cavalrymen and march them near an enemy formation. Attempt to stay as far away from the regular units as possible, while still staying close to the units that you want to kill. Order your "assasins" to attack the drummer or officer in the formation(whichever is closer). Keep shooting until the unit is dead. Most likely the units of the formation will reach your troops before they have time to retreat, and they will have to die. That is why one must keep the amount of soldiers attacking at a minimum. The death of either the officer or drummer in a formation will cause the formation itself to dissolve, lessening the units' abilities to fight and defend themselves. This makes them very open to a frontal attack.
Assasination Antidote: If you believe that assasinations may be conducted, try to have quick cavalry units(like Cossacks) near each of your major formations, so that they can attempt to disengage the attackers before they kill your drummers or officers.
City Raids: This is a highly damaging tactic, if successful. It involves sending a small group of very fast cavalry units(such as Cossacks) into an enemy city, though an unwalled section. When they reach a group of peasants or buildings that are not guarded by soldiers, they will automatically switch ownershing, being owned by the "attacker". If there is little chance that you can send enough troops to these buildings/peasants so that they can be held, simply select the units and buildings and press "Delete". Doing so will destroy those peasants. Although the peasants will be automatically destroyed, the buildings will first lose a large percentage of their hit points and will "catch on fire". With their hit points steadily decreasing, eventually the buildings will explode when the number reaches 0. This can destroy whole cities and ravage a players economy.
City Raids Antidote: Although city raids are highly damaging, they can easily be combatted. One way to stop city raids from being successful is by "garrisoning" troops inside your city. This means that you should station many cheap units all around your city, keeping the raiders from capturing your buildings or peasants. If you do not wish to spend the food and gold to garrison your city, you may build a large stone wall, closing all "points of entry" into your city. Only artillery can easily raze these buildings, so it is near impossible for raiders to even enter your city, thus preventing them from capturing your units and peasants. Though, if you did not take these precautions and raiders are about to capture your units/buildings, simply select your buildings/units that are about to be captured and press "Delete". It will kill your peasants and greatly damage your buildings, preventing your enemy from gaining anything, though it will damage yourself.