See the post above; I think I explained everything in detail. Let's just say the mission is challenging (for the hero) but not impossible...

I advise you to level up the Human Castle more because if Markel gets a decent upgrade, ideally right away with Fire Warriors, if that doesn't work out, then at least by level 2 in the mission.

Since both Castles are mainly resource gatherers, you won't hinder them. Markel won't be bothered until you enter the underground...

I explained how to quickly defeat the archers in the first week; you should experiment with griffins and the "Strike from the Sky" spell...

Markel, with his single vampire, can easily take the mine. Even with a weak build, a single vampire is still effective, although it's better to replay the first mission and use Fire Warriors; it will be much easier...

As for Fiona, machines are a must-have, as mentioned; the stacks are strong. I was lucky with the hut on Fiona's territory, and it gave me light, although there were still no mass casts...

I advise you to make a quick save and plan Fiona's build. Keep a notepad and write down the build. If you don't like it, restart until you have an optimal build. Once you have at least basic training halls, train 7 archers and then 20...
When Fiona reaches the maximum level, you can start the companion, who will stand at the portal from which the demons emerge, while Fiona goes further...
And one more thing, if you really try, Fiona and Markel can go to each other's territories. After defeating the first demon Castle, Fiona gets a portal to the city and can stand in a specific spot on the map and teleport to Mythgard. If Markel has a scholar, he can learn the spell and teleport to Claw, thereby boosting our main heroes, and thanks to this, the 3rd mission will seem like a walk in the park.

Good luck! If you really want to, you can even go to space; I'm not talking about the player, but I think someone who has completed the Master Campaign knows a thing or two.
Good luck!