Resurrecting Grandma would still cause more damage, because she can attack ICE HYDRA, Fooruman can attack Ice Hydra, and everyone else defend.

Since Radium cannot afford to let Grandma live, she will be killed, leaving Mythslayer to cast Slay, then attack either Jim or 21HP Ice Hydra. Finally, Araches/Leetsan will be free to cast Poison.

We could even take a risk and attack Solstice with Mythslayer right now, because the hydras and Solstice will be busy killing Grandma and Fooruman.