Dragon Age, in general, has easy bosses. The trouble is, while the bosses themselves can be readily dealt with by kiting if nothing else, they tend to come with gobs of support characters that make the battle difficult. Once the support is dead, if your tank can't absorb the damage (or is dead) or your healer can't heal through the damage (or is also dead), then simply switch everyone to ranged weapons and manually controll the target character and run the boss in circles.
In one case it was even easier. There is a particular Dragon that, when fought, simply stands in one place and will not pursue the party. At range it doesn't have a lot of options - a fireball (without knockdown), a mass stun and a move that allows it to pull a single party member into fang range. While up close the dragon is absolutely brutal and can readily tear any party member to pieces regardless of healing capacity of the healer (unless you engage it late game, which is unlikely for most play throughs I'd suspect), from range it is less dangerous that a Hurlock Emisarry, with the one exception that it has a dramatically improved HP pool.
Likely the bosses that proved most difficult were the revenants when trying to gain the Juggernaut armor pieces as each revenant had pulls, mass pulls, brutal damage output, thick armor, loads of health and came complete with a number of lieutenant grade monsters and a handful of normal mobs. If you're playing on a harder difficulty, it takes a fairly late game attempt to make it through without losing at least one of your characters in the fight. Your tank will be busy trying to hold the Revenant's attention, and the rest of the characters prove a hard fight for most of the rest of the party. Since they are undead, they tend to be highly resistant to the early stuns you have access to (cone of cold for example), and they also put out high DPS. Since most parties only have one actual tank, it's generally left to your squisiher rogue to try and hold the attention of the rest while your healer will be having to pull double duty keeping the actual tank alive and the rogue.