So I just beat this level five minutes ago after trying for around 4 months.
I destroyed all enemies but two Zombies and one Dragon cannon that were originally positioned at the bottom right of grid. I had only my Hero left and he destroyed the opponents castle wall. Battle was won with two stars.
Hero: Behemoth (LVL 8) Earth Medal (LVL 2) HP 930, ATK 24, D 7 (Equipment Off) HP 1430, ATK 51, D 13 (Equipment On)
Equipment: Willy's Helmet Crystal Hammer Armor of Fury Willy's Shoulder Pads Wristbands of Fury Royal Clover (Ring) Hand of Wade (Ring)