"Wishing Well" is super, but a little bit to easy.
I tried to defeat the enemy keepers with a dark angel, which I succeeded in doing. However, I did not pay attention to the fact that there were other battles apart from this main one, in which I lost almost my entire army.
My army before marching to the king:
1x Dark Angel - Level 10 (There were a hell of a lot of fights he did on his own, hence level 10.)
1x Giant - Level 3
1x Wizard - Level 5
1x Dark Mistress - Level 7
2x Salamander - Level 5 + Level 6
4x Bile Demon - Level 1 + 2x Level 5 + 1x Level 6
I've rarely had a smaller army than this. But these few fighters (and a few lightning traps) forced the king to the ground.