The Medusa did, in fact, see Frisk poking their head in earlier and has been patiently waiting to strike the instant it caught movement. As the door slams open, it immediately surges forward with startling agility, each snake head poised to sink their venomous fangs in their target.

It was not expecting to run smack into Kanji's hammer. Though the force of the collision sends Kanji tumbling back, the same goes for the Medusa; the monster staggers backwards with a shriek, teetering at the edge of falling over.

This gives everyone more than enough time to file into the room. There's plenty of space to maneuver as well -- just don't wander too close to the walls, or you'll find the water is not as shallow in the corners as it is in the center.