I've played as Willow for quick some time and as Yuutang has said she places her fire down even if you're moving and right before the grue attacks you. However it is possible to be killed because the grue has hit me once. Granted its only happened once so it's extremely rare it is possible. Willow only places her fire down in total darkness. I believe the reason I got hit was because I was weaving between my fireplace and into darkness and it threw the timing off somehow.