It's really hard with a Hunter as you have no base Void damage...whereas a Warlock and Titan both have a void subclass with void grenades and void melee abilities. Just to be clear, any kill you get with a grenade or melee will not count towards Thorn, and if you trade kills you'll actually be -2, as it's: +5 for void kills, +0 for any other type of kill, +0 for any assists (even if void) and -2 for any death. Maybe hold off until you have Atheon's Epilogue or Word of Crota to use as primary weapons, a void shotgun, fusion rifle or sniper (depending on your play style) and a void rocket launcher (ideally Truth). There's no point using your super even, unless it's to save yourself from dying. I quit the bounty on my Hunter and waited to get it on my Warlock. Even as a Voidwalker with Word of Crota, a rapid firing void sniper and Truth it still took me 14 hours :/