Easy on low settings. Keep your pistol at the beggining, STAY AWAY FROM THE GUNS. Just make sure you pick up another pistol along the way. Use Pipebombs when you have them, and Molotav-ificate Tanks and Witches. Try to crouch behind the AI and shoot from there, as they get amazing shots all the time. The only part that may be troublesome is the finale.
No Mercy- Set out the gasoline tanks outside the ramp and hide under it. the AI should follow and its just wash rinse repeat.
Death Toll- Have the AI follow you to the glitched rock and just stand on it and the AI should knock out most of the Special Infected before they are a threat.
Dead Air- Sit on the hill with the AI. You get amazing shots here so its absolutly no problem.
Blood Harvest- Stay upstairs, close the doors. Put any explosives (gasoline or propane tanks) at the bottom of the stairs. Be sure only to shoot them when the Tanks come. Pretty easy with the three amazing AI.
Hope this helps! Leave any questions as comments or PM me!