Just wait till you accidentally stumble upon the three in empris du lion! 19, 21, 23! Those are rough.
Generally speaking though, if you're having trouble with the level 12 dragon in Hinterlands, and you're higher level, you need to go back to skyhold and re-evaluate your equipment. Double check how you've speced yourself and your team, how you've geared them. Resistance gear is important for pretty much everything past Normal setting.
Make yourself and your team members better gear, and make sure you're bringing a tank. My general go-to for Dragons is two ranged (Mage and either sera/varric/myself if I'm a bow rogue) and two melee (Cass or Blackwall as tank, Ironbull or Cole or myself if I'm a 2handed war or daggers rogue). The only time I've deviated from that setup was when I was a mage myself, then I brought a second mage, a bow rogue, and a tank.
And, if all else fails, come back later. Or lower the settings to casual if you're not already there ;p