Start off with a stack of your Hero + 2 Initiates + 1 Ghost. Or you can go for 3 Initiates if you picked the Nosferatu and have buffed his armor (10, Banner + 20, Natural Armor + 10, Runestone + 15, Holy Chalice = 55 Armor!)
Once you get Wraiths, remove the Ghost, either move your Hero to the back line or leave him in the front (preferrably in a side slot) add 2-3 more Initiates. Although I don't recommend leaving a Lich Queen in the front line, she'll end up losing too much HP and eventually you'll run out of potions.
If you're using a Death Knight then... yeah good luck with that, I've never bothered using Fighter (and Archer, except for the Nosferatu of course) Heroes in the sagas. So can't give any advice on that. I suppose you can still try spamming Wraiths. Try to shift your DK around the front line so he won't be receiving a ton of damage, seeing as he'll be useless in the backline