In regards to defense only, yes slash makes it seem meaningless, however… Can her skill not be considered a hybrid defense buff? Regardless of color spectrum…
What Is Elemental Damage? - A Guide to Elements
So… if you have a rainbow defense team with Norns and she fires her skill: Someone on your crew just went from taking double tile damage to half tile damage.
And since she hits 2-3 enemy… depending on the enemies attack (3-2 / 2-2-1), potentially multiple allies receive protection from strong tile damage.
I agree that’s not the best defense skill ever but… it’s better than Margaret! 