Considering that Strange has been in this fight for a lot less time than Steven has, he's quicker on his feet than the gem. As soon as he spots that coldfire, he poofs into smoke and the beam passes directly through him. It feels odd as hell but he's not any more broken than he already is.

This halts the growth of his vine/earth/plants mixture...but as soon as he reforms, a few feet away from where he was originally, Strange starts murmuring, regrowing the vines and earth, and trying to hold the Mother of Invention down some more.