In the guide for High Five (getting a combo of five), I explain how the combo counter works. Basically the skulls in top right corner appear when you kill an enemy and the ones crossed off are your kills in the combo. When it reaches five skulls crossed out, as in five enemies killed in the combo, the skulls will reset but now the first skull will be purple.
To get this achievement you will have to have the first three skulls be purple by killing 15 enemies before the counter goes down. This is fairly easy to do with the shotgun when you come across the big rooms with lots of enemies. You can jump on the walls repeatably and shootout when you are sliding down to take out most of them.