It should be still 15 kills. A common mistake that many people make is right at the beginning of the fight. When you shoot the first Ganado you have to make sure you have the correct distance to him. Look at some shadows on the ground, they should mark like a big X. Thats a good visual sign on where to stand. If you are too close to the guy, the next wave will spawn in a different spot. So you will kill less enemies with your grenades. Only thing I can think of why it doesn't work for you.