Therefore, I'm posting another screenshot; I think this method is known to more than just me.
The idea is to find an enemy creature that attacks your warrior sufficiently, and at the same time, the mage can heal it at the same rate. As a result, your Ignez doesn't die, and the mage raises an "astral."
So, in the next mission after my previous post, I decided to level up an astral:)... A long-lived ogre wasn't enough; it has too poor regeneration. It's better to wait for a troll and level it up calmly, as MasterPainKiller described.