I have been able to kill all of the monster dens with no loses and without using any healer units all the way until yetis. My normal army would only consist 3-4 sages anyway so it's still possible even without any nerfed magical gandalfs. Charriots would actually work 10x better because of the 2 range and they can tank more damage until you get better front line units.
Basically you are not asking for a fair play, you are asking to bring the loophole back which allowed you to get some free experience and loot from monsters without spending anything. This is not going to happen.
There's always going to be a stronger formation that would be able to destroy pretty much anything and that's because of how the current monsters are programmed.