The best stage to pick for this achievement is the HARD RAIN campaign chapter 5 TOWN ESCAPE . When you start in the safe room there is always a defibrillator for the SHOCK JOCK achievement, really good weapons, and most of the time there will be an adrenaline or two for THE QUICK AND THE DEAD achievement as well. Also if you have an extra controller and sign in a guest you don't have to try and lure the bots out of the safe room. Change the difficulty to expert and kill the other survivors and leave your guest standing. Then take the guest out of the safe room and shut the door. Wait for it to die and be sure to grab a defibrillator and an adrenaline if its there. Change the difficulty to easy and kill the zombies but try to to leave at least one alive. Then revive your guest and go back into the safe room. Sometimes another hoard will come so you might not need to change it to expert but if it doesnt you can change it to expert if you want to. Make sure your change it back to easy before your guest dies you want it alive but incapped. Go out and clear out the zombies but if you have another adrenaline in the safe room try to leave a zombie alive so you can do it again. Then take your adrenaline shot and pick up your guest. Once you have used up your adrenaline shots change it to expert and kill your guest or find a zombie or you can go over the downed fence through the gas station and jump in the lake its an instant kill so you can start the chapter over and do it all over again.
This was the way I found to be most reliable to get both achievements quickly. Any questions send me a message via xbox live. GT LBCeroy