If it is trying to break contact and target your undead, you are going to need to focus fire it and take it out asap.

If the wisp isn't trying to break contact the buffed unit should easily handle it. My advice would be to buff a human or wisp with nor's boon and then rush that unit into melee with the guardian wisp to try and lock it down and keep it from attacking your undead. +50% white resistance and attacks death damage. Undead mages amplify the above strategy since they have Nor's Boon.

Having a mortal unit in your stack has other benefits as well such as the ability to cannibalize if you are desperate for healing after a battle. If you aren't in an area where humans are easily hired you can always rely on a wisp, while they aren't resistant to it, they aren't weak to it either. Humans are your best option for countering white damage since they have a base 20% resistance and can get more while leveling up. If they can't be avoided, then you will need to adjust your stack so that they can be countered. Go in a different direction that has enemies you are more equipped to deal with. The best option is to simply avoid them until you are better prepared. Anything dealing white damage can be a big threat to us early game.

Free SpellForce: Conquest of Eo