When playing as her, I usually use the speed-up spell on my first turn, and then, depending on the amount of mana, I use either a regular lightning spell or a chain lightning spell (I rarely use area-of-effect spells because the witches’ ability doesn’t work, although against Jaliba, it’s the only option).
I’ve encountered a couple of Erin-Knight matchups, and I was playing as Erin. In general, by the end, I had witches left, while the Knight had either archers and some heavy unit (Griffons and/or Inquisitors), or a siege engine and some heavy unit, and then the Knight casts Slow, which Erin can’t remove...