Hey guys,
i just got the game few days ago and i'm having a lot of trouble:
in past two games i was able to play and win everything (at Nightmare of course) with a "standard" party setup, i mean with tank+healer+2xdps, however now i die to almost every 4+ enemy group (specially those with 2xdamn lesser terror demon) and i have not idea what's wrong.
I see and understand very well the new Guard, Barrier and drink-potion mechanics (i'm running a Support mage with Barrier -Solas- instead of Healer), problem is my Tank doesn't hold aggro for more than few secs after using the taunt-skills (already got War Cry+Challenge+Charging Bull, however isn't enough lol), so my party is aggro-ing everything and die, i'm always the last survivor till the death some mins later 
That's a real pity because with Guard+Barrier+Potion i can handle a lot of damage and almost never die! (got also all guard skills: Charg.Bull, Unt.Defense,Unbowed,ShieldWall,etc)
Now perhaps trying to understand what i'm doing wrong is useless, just tell me some tips on how i should play this game would be easier for you 
Thanks!
PS. i'm playing with Hard difficulty, Friendly Fire ON, tank+Solas+IronBull+Varric, level 9-10, i die to every group enemy lvl 9+, allied with Templars, next quest: close the rifte at Haven.
Turn FF off for your first playthrough. That's the easiest thing to do. Learn how various skills work together, not to mention the tactics of enemies and bosses, then try FF on a second PT.
Just a side comment: the concepts of "tank", "support", and "healer" are very old school. They don't really fit in DAI. Every class has a way to attract or evade aggro. Every class deals damage. Every class can do some kind of healing, albeit indirectly.
For example, in addition to War Cry, you also need some crowd control skills. Gone are the days of a single tank aggroing everything on the battlefield. Subgroups or individuals of the enemy will reposition themselves on the battlefield, outside of the influence of War Cry (which has a fairly small radius to begin with). Enemy archers and Terrors are particularly good at this. That's where skills like Chain Lightning, KO Powder, Grappling Chain, Wall of (element), etc. come in. Rift Mage has the best assortment of CC skills, so run with Solas everywhere for your first PT.
Evasion is also important, at least for low to mid level. Every character should have some kind of "Oh Sh*t" button to get out of a bad melee situation.
Finally, you need to spend a significant amount of time crafting gear. If you aren't doing some min/maxing of mats to gear, you're going to get your a$$ handed to you. I should know, I'm intentionally doing a no-crafting NM run in order to up the challenge of the game.
This is important with FF off, but with FF on it is vital.