Great little game, but... Machine Gun + Ice Tower + Rocket Launcher + Upgrades = Win any level.
Needs some tweaking on the power of the towers, i guess. Great, nonetheless.
Two last battles are fairly easy.
The End = send titan units / dragons with high HP to stop him, and kill him with long range units + arrows.
King Of Hell = use elves, send lots of titan units and red dragons. NO Angels, dryads or archers, magic heals him.