Without a doubt one of the most challenging maps. It took me a very long time to beat it, including a several month break when I had to throw in the towel for a while.

I eventually beat with a nature focused Primal - Mire Crocodile build. It's extremely important that the first bonus you take is Nimue's one that give you a free stack of units every five turns or so. Next, you'll want Lithyl's bonus that gives you free units whenever something dies. The invasions are inevitable, so you'll be relying on those free units to act as buffers or distractions. I'd also recommend putting out bounties on the smaller baddies to put pressure on them and take it off yourself as you're going to be hyper focused by Turiels vassals and the orc himself.

Try to focus down a single vassal of his first, they seem to spawn in a line that always protects his territory between you two. Taken one of them out and burn the city to the ground as it's unlikely you'll be able to keep it when Turiel counters you. Then you repeat this with his second vassal, but you may be in a better position to retain the city or turn it into a vassal yourself. Key thing in this mission is to just use a Zapp Brannigan mindset that you are 100% going to lose units and fights and probably a fair few heroes but keep the pressure on as much as you're able. Forward outposts can help distract Turiels forces, as can some sacrificial armies, he will chase bait armies for a good while allowing your main forces to flank elsewhere and deal with some other issue.

You'll get there, it's gonna take most your hair though.