Here are general tips for playing the Empire Campaign in Warhammer: Mark of Chaos on HARD Difficulty. A list of all our Warhammer: Mark of Chaos articles can be found here.
We won't be hand-holding you through absolutely everything, but instead just give tips on situations that we found tricky or that you might not have considered.
Chapter 2: The Hosts of Ulthuan
Mission 4: Warriors of the Night (optional)
There is a Jade Wizard available, but do not buy him yet. Try to get him when you have maximized Von Kessel's level prior to the final mission in this Chapter, since heroes start at the level of whichever main character buys them; so the later you get them, the higher their level without having to earn any of that XP.
This mission can easily be won with just the Captain and the Bright Wizard. Nothing really tricky here.