Dungeons 3

Done in the "Everything Has An End…" level. In 3 specific areas are Heroes who are being threatened by Tanos. The below spoiler tag shows the 3 locations.

To save these Heroes Thalya must go near to a circle of fire blocking their escape from the encroaching flames and use the Frost Nova special ability to put out the circle of fire. If done quickly enough, the Heroes will automatically flee the area and join your army. They are strong fighters and do not add to your total creature count so will come in handy for defending your dungeon. Once the 3rd group of Heroes is saved by Thalya the trophy will pop. Note that the Narrator will signal that you're near one such group that needs saving and from then on, you will only have a few seconds to locate the circle of fire and use Frost Nova on it to free the trapped victims.

The following image, shows the location of the 3 groups of heroes:

For this trophy, no Hero can die to the flames. Once the last group is saved and every Hero survived, the trophy will pop.