So, I can't comment on this decision. It seems that hypnotized and simple-minded creatures won't attack each other... although I'm not sure.
In general, I'm writing this based on how I did it :).
The secret to the task was in the word "break through," not "win."
Here's how it went. We buy Septienna (specialty: Wave of Death).
We buy Labetta (specialty: Stone Skin). Septienna teaches Labetta the Wave spell.
Labetta, with 1 Pixie and 1 Skeleton, enters a +1000 defense stack. She gains +1 skill and advanced earth magic. Now, the Wave spell hits her for 40 damage. We attack the elementals (by the way, this was on grass). The magic creature runs away. The Pixie draws the elementals away from the Skeleton. On the Skeleton, we cast the Wave spell (40 damage). The Pixie dies.
Turn 2. The elementals can't reach the Skeleton. We cast the second Wave spell. Only 1 magic elemental remains. We run away.
Since only the weakened stack remains in the army, and it disappears when running away, it means that there are no elementals left on the map. Conclusion: we broke through the elementals :).