Last time I checked this only works for very slow-moving targets. Like Urrox the flame prince in PoE, any other enemy will just walk outside of the radius since they're programmed to do so. I've never owned a Gjally, (day one player with the worst possible luck) so the rocket that have taken that role have mostly been Hunger. But I've used Dragonsbreath on a few occasions, found out it works best for slow moving targets which there are very few of.