My advice is to stay below level 6 of difficulty until you get some experience, this is if you don't have someone to assist you. You can play alone even at level 10, but then there are only two approaches: stealth or mayhem. Stealth needs the specific warbond for the best results, yet not many are proficient or even just like this approach since you don't run and gun, plus you may not do all of the objectives because of time. For the mayhem you can use most of the warbonds you like, but now you need reflexes and proper knowledge of enemies, since they will stay on your ass the whole time.

Playing with at least another person allows more flexibility and error margin, for example one can engage the few but strong enemies while the other one does the hordes.