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This spell is most useful either as typical AoE if stronger options are unavailable or to give your surrounded single-hex stack some breathing room. Psychic and Magic Elementals and Devils are some of the units that do well with such tactics.
As it is frost-based, many fire-immune creatures will take damage from this spell. Fire and Energy Elementals are vulnerable to it, taking double damage.
Keep in mind this spell will damage stacks occupying all 6 hexes apart from the central one, unless they are immune to magic, immune to this spell, or resistant to magic damage. Take care not to hit your own troops if they are not protected in some way. Army boasting such troops can use Frost Ring more aggressively.
Units immune to Frost Ring:
Heroes with Frost Ring as a starting spell and a specialty:
Creatures vulnerable to Frost Ring