

To dispel it (Chant of Unlife) or any others like it, you have to have enough casting and mana, and when the battle starts, just click on the picture top left of the screen that represents the effect, it's kind of a coin toss if you are successful in dispelling it

If you are speaking of the one where they spawn a new one each turn for example spiders, then you just have to kill them all, that one requires you to be a bit more powerful, you just have to keep your units together and support each other, things like heal, and regrowth are huge help for those The idea is to stand on the dead, so they don't come back to life and don't outnumber you.
