The sounds are more than enough to tell Greg that it's not working. Even with Greg's help and the jumbo-sized blasters, it's not enough to do any more than hold Blanche off. There's only so long that'll work.

For a second, Greg can't move. The bubble that's protecting him keeps him in this room, unable to work any knobs or even fit through doors. Blanche is going to kill him for getting in his way, it's better for him to stay here, stay safe, minimize the damage he's done. Outside, the roar of laser blasting rattles his door on its hinges and makes him shudder.

The bubble pops, suddenly.

Greg sucks in a breath, and runs for the door. They're going to wreck this place and wind up dead if he can't protect Sans.