Btw it can be done W/o the mermaid, (as i don't have it and don't feel like going thru the game a third to get it :D)
i used Dwarf engineer Fire Succubus, Goblin Soldier, Bomber, Taurus, Black Knight. Skill Cards=Freeze trap, Pheonix Strike, Lightnig Bolt, Healing Wave, Hercule's, Athena's.
I waited till all my units where near Max number, keeping the captian out to take care of the guys, and swarming for a moment for him to heal. when it was time to go slightly before half way i unleashed the Pheonix, by the time it reached the other side you should be near there. then i used freeze trap and once that let up then Lighning bolt. then Athena/Hercule and let loose. he stayed up at his castle the whole time and i was immune for the 1 shot. he got maybe 1 small wave down before the castle was destroyed. i used the Healing Wave for when it was time to charge in just to be safe(and only thing i could afford anyways :D)
give it a shot for urself and good luck!
 "Epic War 3"){kind=icon}