Trickster achievement
This battle is very boring, it is really difficult.
I lost several times, tried several combinations and finally found a strategy to overcome this challenge (hard mode).
I refer to the first challenge of the genius, where we face each other.
Strategy:
Use only the Apprentice and the Elementary (I had a few units of each and that helped make it faster).
The Apprentice barely fights, avoids battle and, whenever possible, uses his incantation skill.
The Elementalist will be the main character. And the rival Elementalist will be your first target.
Use Ice Shard to damage the rival Elementalist, start your ranged attack and use the Eartquake ability to stop the rival Apprentice from advancing.
Keep these movements, always avoiding hand-to-hand combat. The rival Elementalist will soon die.
Use Cure to renew your Elementalist and maintain the same escape strategy and protect it with Earthquake.
If necessary, use your Apprentice as an obstacle so that the enemy does not hit your Elementalist.
If you did it right, you will win without problems.
Here I put this strategy to try to help someone in need. I lost many times in that battle and that was not pleasant.