I want to thank you for this strategy. It actually worked, though I did not follow it completely, sometimes improvising. I also did not do it at first try, but it took me around 4-5 tries and I have achieved victory. I have also done it with lv. 1 difficulty, and even so I still had a hard time at the start, when 2 waves were approaching. From personal experience, I want to say that everything past two waves of enemies was not that difficult. This is my setup:
lvl 18 Orca with 220% max hp buff and 180% defense
lvl 13 goblin mage
lvl 14 centaur
lvl 11 centaur
lvl 13 kitsune
lvl 12 kitsune
lvl 14 mushroom girl
lvl 12 Alraune(flower girl)
lvl 11 flower girl
I also have 2 more characters here, but they do not matter, as I have never used them in a fight. Also, during waves goblin mage was constantly dying and I have spawned her twice in a fight. And well, I didn't get much profit from her after I have spawned her second time,she died as quickly. Also, centaurs and kitsunes had enough health to survive 1 shot from heavy artillery, so that gave me some time.
By the way, what does the upper buff of swordsman do? I have an error