You might have to team up with one or two people then, what level are you at and what character/subclass are you using?
The captains are definitely a pain in the ass, if you get past the first wave to where the witches come out sit back a bit and have the hive and fallen fight it out (the fallen eventually lose from my experience for the same reason you listed with the captains) the witches will damage the captains.
As far as legendaries go the vanguard sells a sniper rifle that does arc (epitaph 2261) but you need to be vanguard rank 3 and I believe it's 150 marks as well. It's a solid gun if you can afford it and is what I used for the weekly and nightfall with my Titan.
The captains are definitely a pain in the ass, if you get past the first wave to where the witches come out sit back a bit and have the hive and fallen fight it out (the fallen eventually lose from my experience for the same reason you listed with the captains) the witches will damage the captains.
As far as legendaries go the vanguard sells a sniper rifle that does arc (epitaph 2261) but you need to be vanguard rank 3 and I believe it's 150 marks as well. It's a solid gun if you can afford it and is what I used for the weekly and nightfall with my Titan.