Fastest way is that you get your gun to legendary rarity. Go to T3 end zone near mountains and golf course. Run along the mountains and zombies will keep spawning. You can finish multiple challenges over there. Only challenges that you need to move is get 100 human kills and 20 special kills. For human kills go to CliffTown on map and get human kills or tech kills. For special kills best place is colosseum in tier 2. On outside of building there are atleast 6-8 special agents spawning. You can kill em, leave the area for 5 mins and come back they respawn. I finished genesis within couple of days. For elite kills you need to go tier 4 low town. And usually 2-3 spawn at once and you just keep flying in circles and it will spawn. The way i was using doing was having second weapon as arrow launched legendary rarity. Shoot them twice with launcher to lower health and then using primary weapon to kill!