Death Borg Nu is one of the weakest borgs in the entire game, don't expect him to do much other than die or do a tiny bit of damage. Avoid fighting long range type borgs, as they will easily kill you in one or two hits. Your best bet at being useful is shooting at enemies going for your allies, hoping to interrupt them for a short time so your ally can recover. Try and take defensive positioning, hiding in high or hard to shoot to places. If you're lucky enough that your ally picks one, try and stay ontop of a Fortress borg, seeing as you'll largely be ignored as people go for the big guy. You can also be somewhat useful as a mini turret if you get on an Air Borg. You should generally stick to your allies, and never go after other borgs unless they're about to die.