You don't actually have to kill 500 Guardians necessarily, you just have to get 500 points. A kill with void damage is 5 points, a death takes away 2 points, so you just have to get over a 0.4 kill to death ratio while using exclusively void weaponry and you'll eventually clear the bounty. Thorn is pretty thematic and strong, and I feel the challenge is very doable if you have the right strategy. Rumble (because of the chaos and the every man for himself and lack of general strategy / preservation instincts) or Control (camping control points) is the best way to rack up kills, bring a solid void fusion rifle / shotgun and a void heavy machine gun (and Atheon's Epilogue if you have one and can stand the darn thing) and focus on getting kills over winning the game. Control the heavy spawns, learn the maps, be slightly more clever than your opponents. Playing with friends is also highly encouraged. Good luck!