To be honest if you have a fire team it's not all that hard, getting to the dark blade is a challenge. But that's it really, when you get to the dark blade all you do is, keep alive the knight that corresponds to your sword element let's say arc, and kill off the void and solar one straight away. Then DPS the dark blade until he is on a sliver of health, when you are at that point kill your knight and have the other members of the fire team kill the dark blade and you are done. Little tip don't bunch together because you can get one hit by the dark blade and get taken out. My fire team found this out the hard way. Lol