Simply throw a flash grenade at a licker to paralyze a licker's sense of hearing and get the "Keep Their Heads Ringin'" achievement. For example, you will encounter a Licker in the corridor leading to the S.T.A.R.S. Office on the 2nd Floor of the Police Station. There is a flash grenade inside the S.T.A.R.S. Office -- so it is a perfect opportunity to get the achievement early on. Lickers are the enemies that crawl up walls and attack with sharp claws and whip around their long tongues.