I have no idea what I'm doing

[BB Shockwave]

I have no idea why you have problems.

The main trick and difference between KB and HOMM is that the Adventure map is real time. This means, you don't have to fight everyone - you can lure them away from the treasures/recruitment places they are guarding. For example, to do the quest that gives you the Rage Box, you'd have to fight your way through to the Observatory in HOMM. In KB, just wait until the enemy armies move away, or lure them away. Then double back, and voila, you have Xp and the Rage box. You can get Sleem for a few snakes then, and Zerock after a battle with a few mage units (not a hard battle). Life is much easier after this.

I usually go and fight the turtle on Level 10 - I leave all armies I know I cannot defeat without losses behind, you will find some easier battles on the Freedom Islands. Again, the trick is to evaluate the enemy and fight only whom you need. Also there is a good chance that you can get Resurrection or Sacrifice on the Freedom Islands.

To defeat the turtle you will only need a strong Tank (a Knight will do, or an Ancient Vampire) and the Healing spell, plus lost of ranged units. It's a breeze.