Theres simple tactics to follow when fighting the dragons and I rarely need to use resist potion or gear after about level 15 and I beat them all. My best advice is to manage your team in Tactical view for the most part. Use resists on your tank or and other character but I found that I could better utilise the potion spot with something better.
Get the inquisition perks that give you more potion slots with greater numbers asap.
Way I fought the dragons is with the same team evertime which consisted of me (Rogue/dual blade/Assasin), Tank=Cassandra, Sara ?(Ranged), Vivienne (Default). With the mage slot you maybe best to use frost/fire mage depending on the dragon you face but I never had too.
My tactics were to use the tank to face the dragon of course and build some aggro then I moved my rogue behind, sara and vivi to oppersite sides of the dragon, dont keep them bunched together. Rogues are capable of massive burst damage but also can die fast so I had Hide in the shadows ready and as soon as the dragon notices you (Text prompt on screen), Use hide and wait a while until skills cooldown and repeat.
The dragon will do a wing flap that drags you in and its un-resistable. I found that my rogue seemed unaffected by this and I could continue to kick butt but sara and vivi took a knock from it and as soon as the dragon does this, switch to tactical and move them as far to the sides of the dragon (Out of its line of sight) as you can and they will keep trying to move away.
Theres another special attack that a dragon does when it goes airbourne, best thing to do is move all your characters to the extreme sides away from each other and keep your tank in the middle (Do it in tactical and with group turned off or passive mode), Now switch to your tank and stay around the middle of all your characters and as the dragon fires its attack at you, dodge out the way and wait for it to land. If one of your characters other than your tank does get hit then having them all away from each other helps of course.
Potion management can be tough but theres a little trick I learned but requires you get the upgrade to the HoT (Heal over time) pot (Forget its name). What that does is allow within a certain radius of the using character, all characters to be healed. So lets say Vivi takes half health damage and so does Sara, use a HoT potion on Vivi and move Sara to Vivi (They can continue to attack but keep them together) and they will both now be healed for 1 pot use which effectively doubles there usage. You can do the same for your tank and rogue if you use one or 2 tanks/melee.
The dragon in the Hinterlands can be annoying but another trick is to use enviroment to your advantage! See that big ledge? duck and cover is the key 🙂 When the dragon takes flight (Area where you fight it, not encounter it the first time, use the large ledge as those area of effect fireballs can be lethal and s/he cannot get you under the ledge. Remember to switch to passive and move all your squad except tank under the ledge, use your tank as bait (Remember to dodge the fireballs).
Remember. Tank front, Dps Behind and Squishies at the sides and you will become a Dragonslayer!
I used the tactic of the tank targetting the head of the dragon while I attacked its legs because once it becomes less mobile its much easier to kill despite what Bioware said about taking out its wings, I found this much easier method.
Good luck all. Glad Im not having to fight them with all the uncertainty of patch 2 and its buggy mess. I suspect that would be more of a challenge than actually beating the dragons themselves! Come on Bioware, really!
Edit: Yes I know I called Sera, Sara 🙂 My main way called Sara and I CBA editing 🙂