Very simple. Do not do this online. Go to local and play by yourself. Set the game options>player options health double and fast Regen.
Play on stone haven or Stonehenge. I forget the name. In the top right corner of the map if your back was facing the castle, there is a burning 1 story house and next to it there is a 2 story house. Simply camp on the stairs of the 2 story house. Call in your squad mate and move around the top floor so he eventually gets into the corner. When the enemies go up the stairs they will turn and look towards him leaving you to shoot them in the back. I suggest you get the fp6 shotgun as it is a 1 shot kill even on the decimation class enemies. Very easy to do and I got it in about 25 minutes.