dude i was in ur shoes yesterday
basically u don't need to destroy everything in that mission, so only necessary to destroy a few towers and a dragon and even though u have very little health it can be done.
I also thought it was impossible, and frustrated, then it turned out i only had to destroy a bit.
Hide behind the tower, save game, come out and destroy a tower, preferably not getting hurt (or a little), come back and save game. This way it's just down to randomness and persistence!
Use potions if u have to, but u shudnt need to
