Party members can do combo attacks, look up the combos and do that.
Try to close the breaches ASAP
Stay away from harder difficulty zones because they are REALLY difficult.
And have fun! Don't make it a chore, it's a really fun game imo (much better than people give it credit for...people just want to hate on EA and Bioware).
Make a fighter or warrior or whatever they call it and use a shield and make it into a tank; the other classes are not exactly hard to play but a basic tank character is very forgiving and your party can make up for your lack of damage. Plus they're actually pretty fun to play :D
Played Dragon Age: Inquisition on Nightmare and can state having three healers really made the game a cake walk. Found it impossible to lose at all with that set-up. Some would call that exploiting the system but it is not as though mechanically the game was that satisfying or dense to begin with. It's basically a low-brow SP MMO RPG with a heavy dose of politics and sex scenes (will vary on what you want, and some are quite funny too). Pretends to have a tactical mode yet has a lot to be desired in that area. Camera is garbage.
Only aspect worth tolerating Inquisition for is the courtroom choices of chopping NPCs heads off in public, or locking them up for life. I prefer the prior_
Even then to me Inquisition is an absolute terrible RPG with some of the worst level design I've ever seen in a video game. A.I. may as well be from a low budget 32-bit game too. However if one must indulge in the game? enjoying the little it does well.