the level is often a problem. I started gaining levels even in the first mission. You should start with two heroes (the second one for gathering and scouting). In the upper right corner, there's a ship with ghosts; it has many crystals, and to the right and down, there's a mine.

Added 7 minutes ago
I easily completed it.

Added 21 minutes ago
Version 1.7.1
And another tip: replay the first mission and execute Martina; it will be easy to gain points in this mission. The Fortress in this mission will be guarded by a weak squad (a few guards and, I think, that's all, without any auras or griffons, etc.).

Added 5 hours and 51 minutes ago
Besides, there are still 2 weeks left until the Fortress. It takes 3-4 days to get there.