FTL speedblitz is a physical attack. It can be defended from in a variety of ways (comparable speed, great durability, anyone with intangibility powers), and these are powers that are in no way unusual. People to be able to bring large physical strength and speed to survive isn't unreasonable. The expectation that people should need plant-manipulating powers to survive IS unreasonable, there is unlikely to even be such a character besides the one inducing the growth to begin with.