I noticed that Monster only uses spels Wen they have u on theyr sigths... For example: if u click to atack and DarkLord Trought a wall (yes this is another bug) And walk to any direction , and stop as u just see a piece of the monster , he will stop casting spell (like healing) and the other ones.. so u should be able to Hit him with a bow or a wand until he dies...
You can do this with monster that stay stuck on the bushes (another one).