It would be better to remove the level restriction but make the final campaign more difficult. First, this would encourage players to level up their heroes instead of rushing through the map, and second, it would be much more interesting if they encountered stronger opponents, so even against the AI, the cost of a mistake could be very high.
Added 5 hours 38 minutes later
What is the victory condition in the scenario about Sandro? It says to kill Nesher, I dealt with him - nothing happened, I defeated the red creatures - still nothing, I killed the green ifrits (I didn't touch them at the beginning) and still nothing. What's the problem?