Fun, but a bit unbalanced
I found this game fun, and the leader going 'peon hunting' is both a way of keeping you on your toes and giving a source of income when your troops are busy holding off a boss.
One issue I found with this game is the fact that almost all battles can be beaten with 'The Big 3' troops, and they're not the last three, either. If you can deploy the Dwarf (Staying Power), the Elf (Attack Power) and the Rider of Rohan (Heavy, Fast reinforcements) at the same time, they'll flatten more than enough orcs to replace them immediately when they die. That, and they're cheap enough to replace them through peon money on bosses.
The trouble with the last three units (Priest, Gandalf, Gryphon) is that they're all very expensive and move slower than a dwarf. Gandalf, for instance, costs far more than you get from a Dark Sorceror (Easiest boss), and they're even matches. Now, for far less you could get 'The Big Three', and they would not only flatten the sorceror, but also push the battle halfway across the field.
A good idea is after the first ten levels to switch exclusively to pulling in 'The Big Three', and maybe replacing the odd archer. Once you get dwarves, you can drop Hobbits (Keypad 1) from reinforcements. Soldiers are a far wiser investment, especially once coupled with a dwarf or elf. At Level 10 you get the Riders, at which point the Soldiers are useless.
This game is interesting and fun, but the later units are overshadowed by the power of cheaper, more expendable units. I see where you're getting at, making sure the top end units are not battle winners, but you should either a) Make them Faster, b) Make them cheaper, or c) Make them have more staying power (health). As it stands they are too slow, too weak, and too irreplacable to actually comprise a part of a pushing force.