Wellcome to destiny!.
I don't think you did anything very wrong. Is a difficult strike the way is given to us. Early in the thread somebody say "save your big guns for the boss last phase" and that kind of make sense.
I think this boss encounter is designed so you must focus in the adds first, and only pay attention to the boss when he is attacking you. Adds first.
As for the skills. Nobody know yet what to do with them. The titan bubble is obvious, some protection where to reload and play peekaboo. But the damage dealing abilities don't do much to the boss.... or the adds, really. Anyway makes sense to use them to create more orbs. Anything that create orbs means that your friends can super more, so can create more orbs. The orbeconomy is exponential, more orbs, more fun.
No that it was very easy to do this either. The groups of mobs are placed or spawn in areas that make it hard to ambushm them with a well placed super. Or are in roaming areas where you risk your teammates not paying attention to the orbs.
We use to find a safe spot and snipe at the boss, then dance with the adds, but the arenas in this assault don't have safe areas, not even temporally. You could try to use a wall has a shield, then shot to the adds from a corner, but the batttlefield change so quick, you may be getting killed by the boss doing that!. Most people seens to dance around the center of the arenas, that is kind of dumb, but better than getting oneshooted by the boss in a hard to reach area, noibye.
For weapons, your average destiny player have many shotguns, fusion rifles of many types. With the scartity of the beta some players did not really had the weapons they want. People that use fusion rifles had to use a pea shotter. So even people that know what they are doing had touble with the assault.
Is a pretty assault, and possibly long. We don't know how long until we have bikes. Every area that can be skipped using bikes will be skipped. People want zergs and short assaults, and this is neither, is the beta one because showcase some amazing graphics, but nobody is going to favorite it in the future. Your average assault will be (probably) more a bug hunt in tunnels than this weird sprawnling dance.