Please, how can I nerf the summon ice elemental spell?

Although I like the game quite a lot, there are a few spells that seem rather overpowered and sharply lessen the difficulty of the game once they are in my arsenal. The one I would most like to nerf is 'summon ice elemental' which freezes all enemies when cast. What do I need to change in the xml so that when I cast that spell there is a much lower chance (like maybe 25-50%) of all opponents being frozen?

Thanks