I'm playing on normal. Blackwall , Solas and Varric are in my party. I think for the Hinterlands dragon part of the problem is that I speced Solas in storm instead of winter
you can re-spec your characters using the tactician's renewal for around 300 gold i think, from the blacksmith
what about your equipments? and you might want to level up a bit for extra skills
the hinterlands dragon (fereldan frostback, fire dragon) is always at lvl 12, and yes it is annoying, better to spec varric with long shot and full draw (if possible) for long range
the crestwood one (northern hunter, lightning dragon) is at lvl 13, not as much annoying as the frostback, and if im not mistaken it is weak to spirit attacks