The griffin is immortal.

I can't seem to beat the second mission; there are no resources, all the stacks are weak, and I can't develop the Castle without resources. I just run back and forth, and it's not interesting at all. I played for a month, building only skeletons and peasants, and then I gave up and quit.

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

I advise you to level up the Human Castle more because if Markel has some upgrades, ideally with Fire Warriors right away. If that doesn't work, at least level up in mission 2.

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 play around with griffins and "Strike from the Sky"...

Markel, with his single vampire, can easily take the mine. Even with a weak build, a single vampire can do the job, 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 the stacks are strong. I was lucky with the hut on Fiona's territory, and I got Light, although there were no mass spells...

I advise you to make a quick save and plan Fiona's build. Keep a piece of paper and write down the build. If you don't like it, restart until you have an optimal build. When 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 come out, and Fiona will go further...
And one more thing: if you really try, you can have Fiona and Markel 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, which will boost our heroes even more, and thanks to this, the third 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