You can get Backstab Master in less than an hour if you play The Cauldron Base (E5C4) on easy. The covenant are more concerned with the generators than they are you so you can literally dance around them and they won't shoot you (once they're focused on the generator). I got around 80-90 assassinations per game this way.
From the start of the chapter, hop in the mantis and kill the three marines that spawn with you. Once you clear the first objective, you can hop out of the mantis and head into the base. From there, I killed the other two marines and dismantled the UNSC turret (grunts that drop behind it would sometimes get on it). From there, just wait for the covies to drop in and start attacking the generators. You shouldn't lose either one.
Also, sometimes the grunts would start running away flailing their hands. I found that instead of chasing them down and sometimes killing them without assassinating them, it was better to just wait til their attention is back on the generators. I also picked up some fuel rod ammo so I could kill the hunters without much trouble once they dropped in.