Dalen, please...))
And here are a couple of (possibly subjective) tips for this mission:
First, when you capture the second castle (it's roughly in the center), take a short break there, build up your army; all the Inquisitors and enemy heroes will come to you, and it will be much easier to defeat them, and then counterattack...
Second, in the final battle with the Haven, concentrate all your identical troops into one unit; this will make it much easier to control their numbers (resurrect them) if they are separated, otherwise, the Haven will grind them down and you won't be happy... And gather some Liches (they are serious guys) for this; you need to capture a building near the last castle (before the portal to the Haven)...
Well, that's basically it... =) The tactic should be pretty good; I got 99% of my skill from it.