The Witch has a set of abilities not dissimilar to a Daemonhost. I actually started with the 40k AoS Daemonhost's abilities and adapted from there. The battleplan itself is pretty simple, just kill the Witch. However, if they have chosen a decent model for their Witch, this could end up being pretty challenging.
One thing I've been having a lot of fun with in the Dark Age stuff is thematic summoning. Summoning a single skeleton or zombie or even a 1-wound daemon isn't really that big of a deal, but it can be a lot of fun and very cinematic. While summoning isn't normally allowed in Skirmish, I've included several minor summoning abilities like this in battleplans and quests.