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Missirka, this mission is quite manageable. It doesn't have any particularly difficult battles. A well-developed Faidaen is virtually immortal. However, the developers have made it more challenging with some tricks. If you know what they are, you shouldn't have any problems.
1. You can't attack Nicholas before you get the Phoenixes. Otherwise, he might disappear forever.
2. There are several dragon lairs. Each one brings the Day X closer, when 666 dragons will arrive at Nicholas's location every day. Therefore, you need to destroy them as quickly as possible. The fewer lairs remain, the later Day X will arrive.
How to kill Nicholas?
1. After destroying all the lairs, you can kill Nicholas.
2. If you want to complete the mission quickly, you can go and kill him with the Phoenixes. They need to survive. I won't say whether all the Phoenixes need to survive, as I left them in a Necromancer city.
The mission can be completed quite calmly. There's no need to rush, as Day X won't arrive soon. But it's also not recommended to delay.
1. Clear the areas around your cities. Improved dwellings are only built in the eastern city. This is where the units from the western city are upgraded and quickly delivered to Faidaen.
2. Capture the nearest Necropolis, visit the Prophet, and clear the surrounding area. Dragons are the priority.
3. Clear the dungeon. Artifacts that the Prophet wants to obtain are located behind the garrisons. During the clearing process, you will find an exit from the dungeon. You need to go to the surface, kill the dragons, and return to continue clearing the dungeon.
4. After completing the clearing of the dungeon, Faidaen will have a solid army. I had 70+ Emerald Dragons. They are not afraid of Magic Dragons, which Faidaen generously used to bombard his sworn enemies. This is enough to defeat any enemy.
I was able to do everything calmly without using logistics.
I don't have a save file for versions 1.x. The last time I played it was in version 3.0.