Medusa Heads in Hint House

[arrgh6601]

I am completing the first mission in the game and it says this:

Watch out for the Medusa Heads in the tower above. They can be killed if you know their weakness

Can anyone clue me in on whats the weakness? I do not have any shields and they are 3 heads blocking the lever.

Also, the first mission has a circle around it. When i complete the mission, does that circle disappear and circle another mission?