What are you using? Titan? Hunter? Warlock? Titan use defender for void nades, warlock use the void class aswell. I found it easier if I just played my normal game and only used the fusion when I knew it was a guaranteed kill. Save your nade for two or more people. And just run with your auto or scout or whatever you usually use. If you didnt get to the heavy ammo in time try and kill the guy who did.. he will drop it.