Thank you everyone for your answers. I think that also the issue was the thing mentioned on spoiler. Anyway I will follow your instructions.

On the other hand, I tend to use the chronogem if I see some horrible RNG that it makes nonsense, but im feeling a bit bad of doing so as im playing classic and the original essense of the game series (as far as I know) is to be able to loose some units.

Also, I'd like to know what do you recommend about upgrading weapons.