Posse talk:Red Dead Hammer Heads

I am stuck on the undead sharpshooter lvl 5 challenge,i have to snipe and kill 5 undead in 8 seconds can anyone help?ive tried hogtying them and shooting them point blank but that dont work because i miss and shoot the ground,and you have to get the 5 kills one after another to make the 8 seconds,any ideas anyone?any and all are appreaciated