I cleared out all zones with a group of lvl 9s and 10s, the main char was probably 12 with no issues whatsoever. Probably used 1000 mana points but at that point of the game you can get a lot of mana through orgys. Characters dont get stronger past lvl 9 unless they have more than 2 natural bonuses which is super rare or you have scyllas or dragonids.I can finish the story by lvl 15 easy but dragon nests you need to be over lvl 25 to even have a chance and you must have a lot of mana to go with it like over 5000.
The key is to have magically talented people, I had 2 chars with magic affinity boosted to 11 the other two where more melee oriented but everyone knew acid spid, mindblast and healing. If you want to have an even stronger party, either catch scyllas or buy them or the dragonids through the underground trader, they have 18 and 20 base statpoints compared to 14 for humans. If they come with stat bonuses you could make an even stronger party, but I didnt have any issues even with 8 cultists.