Enemies in the original KB always chases you to death you know.
Back to the topic.
I recently try my hands on taking the turtle on impossible with "picked-up" armies only. Most of the time my armies consist of Vampires, Skeleton Archers, Snakes and sometimes Griffins.
1. If you ever find a piece of armor called "Steel Cuirass", buy it. It costs a mere 8000 gold and provides 10% physical resistance. Even better if you find the boots (Flickering boots?) that gives 15%.
2. Since you are a mage, I suggest you get distortion magic 2 and upgrade stone skin to level 2 (I hope you have the spell). That gives your tanker another 30% physical resistance, not to mention a even higher defense value.