[K]

The trick to using Falling Frost is to do three things:

A. Get a bigger Water. The higher your water, the more damage is done, the bigger the AoE, and better chance of enemy units failing the save for Frozen effect(cold effect?). This means that a Water 3 guy might kill a few guys and freeze some more, while a Water 7 guy will kill a third and freeze and heavily damage a third of a much larger area (some will escape, of course, but 2/3rds of the survivers won't survive another hit).

B. Keep your troops on Guard Commander. This gives you several more turns of hitting an army, possibly breaking their morale, and allow for

C. Thugging with Cold immune troops. This keeps the enemy army away from your army so that the FFs can do their work.

Results can vary, but this is a pretty effective tactic.