Thanks for trying it out! Yeah the balance curve definitely is wacky so I apologize that you couldn't beat it (A difficulty toggle probably would've been great here).
Setting up the field with a snow storm before they get out lowers their speed and gives you priority to melt them (if you have found a nuking spell). This is probably the most consistent strat I came up with. And in all fairness, this mechanic isn't explained so it's not too intuitive either.