Ernest's attention was mostly focused on fighting off the tentacle that had latched itself on to him — manually, unfortunately, because the shock of being unexpectedly grabbed and wretched to one side had caused him to drop his wand and it was even now rolling slowly away from him on the floor of the Atrium. As such, he didn't noticed what spell she was casting until it actually did sever one of the tentacles.

Oh, no, don't do that! he thought, but was unable to vocalize anything as he was thrown quite violently to the floor while the brain flailed in pain.