Here's my tips to make hard feel like easy(im almost tempted to go nightmare at this point, but want to wait till my next playthrough so I can do the whole way through). I personally play as an assassin dw rogue and her single target damage is so high it feels like cheating, but I think I can help you out anyway. I use Cassandra as my tank as s/b, Templar, and the tank tree with the taunts, solos as my mage(accept for certain dragon fights that are strong to fire) as rift/fire/spirit, and Sera as Archery/tempest/and the poison/trap tree. I've got the tactics as Cassandra defends Cassandra, solus defends Cassandra, my rouge defends himself, and Sera follows my rogue. I turn off AoE abilities so I can control when they get used. The mages will just fire stuff off left and right and end up out of mana if your not careful. I tend to priortize the spirit abilities for AI use, and and then try and set up the big attacks myself.
I set up every fight like this. I send my tank to the middle of the pack and let her do her own thing. Since you play the tank, you need to get to the middle of the action and throw out challenge and and that bottom left ability from the taunt tree that gives guard per enemy. But what you want to do first is have solos cast his abyss ability. It will suck everyone in close to you so you can maximize how much guard you get. Also, have solos drop a fire mine right in the middle of the abyss. Now I'm not sure how an assassin would be uncontrolled, so you may want to go with another mage for more barrier, or Ironbull. But bull as a reaver is tough when you aren't controlling him. You could also go with varric and use his traps on top of Solus mine for crazy damage.
As far as gear set up, make sure you craft some gear for your tank that has +3 guard on hit from the mastercraft materiel. Cassandra rarely even takes damage in multi target fights. Dragon fights are the only time I really have to watch her, but even then, its not a big deal.
For abilities, my bread and butter is the shatter combo. Since winters grasp can be gotten with 1 ability point in frost, I give it to all my mages. A rogue can set that off for massive damage. But the other combos are fine too. Make sure solos goes heavy into spirit. Go all the way down the left side and get dispel, and the passive that gives mana/stamina regen when barrier is up. and make sure you get revive. I also get the first passive on the right side for threat reduction.
For your tank, you just really need to maximize that taunt tree. Guard is the key to everything. Id say go champion as it has just amazing tank abilities, but templar is great too since we fight so many demons.
Also remember that sometimes, you come upon Rifts that are higher level that the others in the area. That can cause you get waxed real quick.
With all that, I dont have to micro manage much outside of Dragon fights. I normally set up the initial moves and positions, and then just control my rogue. That abyss, mine combo is so effective, Sera can handle soft targets solo, and hidden blades coming out of stealth will kill any soft target on its own as well. I hardly ever take damage. The fight is over before Barrier runs out.