I'd rather suggest the custom challenge present itself as the form of achievements.
1. Let There Be Light
Barracks only completing stage 12: The Dark Tower. Campaign (no lives cost). Normal difficulty.
2. All Mighty Big Bertha:
Big Berthas only beating level 14: Ruins of Acaroth. Campaign/normal. No lives cost.
3. Sorcerer Mages Rock:
Complete stage 14 heroic/normal with Sorcerers only.
4. The Primitive
Stage 3 (Pagras) iron/normal. Level 0 max (no upgrades).
5. I Am Not a Magician
Stage 4 (Twin Rivers) iron/normal. No spell of rain of fire.
6. Two Stand Against Many (from Kilele)
Stage 10 (The wastes) iron/normal. No more than 2 towers.
All of these above have been proved to be doable even without the assistance of the extra features in the Premium, and I hope you guys like them^^