Uff... I finally dealt with the boss. Indeed, with a decent army and some artifact upgrades, the battle isn't that epic... You just spam him with a special lightning spell, and the mages add a few punches... The djinns block the fire elementals, and the boss destroys the rest of the elementals.
Honestly, I didn't like the puzzle – I put in so much effort to get the hero some defense, and then I had to struggle with the mentor to get rid of unnecessary skills, and then try to get something back with another mentor...
I completed it with a Warrior in 5 months, so guys, don't play on Recruit difficulty if you don't want a hardcore experience:D By the way, I got a goblin, and Juss465 wrote that he got a druid after 7 months with a Warrior... That's a bummer;)