The game just has high variance, at any difficulty level. It's possible to have a breeze of a battle against overleveled enemies on the highest difficulty if the gems line up right for you, and it's possible to get obliterated by low level enemies on the lowest difficulty if they line up perfectly for your opponent. Neither of these is particularly common, but both can happen.
There's already a mechanic that stops the enemy from getting a cascade from new random gems in the easiest difficulty to make a long lucky cascade less likely to happen. And in the case that it does, their parameters are halved, making it's less punishing. They are also more likely to ignore any match-4 to get an extra action, unless it's unavoidable.
- Punipen