Dragon towards end of The Nature of the Beast?

What difficulty are you playing on? Have you patched to 1.01 yet? Patching to 1.01 makes normal a bit easier and easy a lot easier.
Before the patch there were some fights on normal that were really damn tough.

What's your party? If you have any mages, use some hexes, cast anything that does lingering damage (walking bomb, crushing prison, etc) and, if you ever need time to get potions properly doled out and some stamina regeneration, cast a force field on the beast.

I have almost the same party setup as yours: duelist warrior, Alistair as a shield tank, Leliana as an archer, and Morrigan as debuffer.

Keep stunning/debuffing the dragon with your Morrigan. Use your taunt if the dragon targets Morrigan. Position your rogue behind the dragon for backstabbing.

I didn't have healing spell then, so I used a lot of portions.

Cone of cold whenever you can that also works on all dragon types

The absolute best strategy for fighting dragons is to only have one character engage the dragon toe to toe. Everyone near the dragon takes damage from some attack or other, so put your tank (with all your best fire resist gear!) up close and get everyone else away. See, it's so much easier to worry about keeping one character alive rather than 3! That's the secret to beating dragons in this game.

Have Sten and your main plink away with bows. As suggested, Morrigan can use her cold spells. It's a toss up between frost weapons and telekinetic weapons. Try to always save a force field or cone of cold in case Alistair gets snatched by the dragon's jaws (in which case he gets shaken like a rag doll until he dies). Both those spells will release him, and I actually like to cast the force field on Alistair. The dragon will keep attacking him even though he's invulnerable.

If Morrigan has any hexes, those are good too if you can get them to stick. Vulnerability Hex will make the dragon take more damage from your elemental spells. Misdirection Hex and Death Hexes are good for obvious reasons.

Good luck!

This may or may not help you. Although this is a smaller dragon, any dragon is difficult. So what i normally do, and this is just me, is to have an archer, a mage(if possible with both damage and healing), a tank, and a damage dealer. Position the archer and mage back away from the fight and have the archer just shoot away. Any chance you get have them do a special attack because they may do more damage or affect the dragon in a different manner. Have the mage do any cold attacks they have because this affects any dragon. This doesn't mean have your mage only build up with cold attacks, although they are useful against all foes. Have you tank attract the dragons attention and try to keep it. This means your tank must have a very good con because the dragon hits very hard. Finally, have your damage dealer just pound away at the dragon. This is the same as the archer; you want to have them use their special attacks as often as possible. Your going to want to have alot of potions coming into these kinda of fights because your mage wont be able to heal all the time. Depending on their con, con affects the amount if damage they can take and how much the potions affect them, you'll want to keep them on the upper part of their health bar. Now,if possible, you want to have your mage be a spirit healer because they have revival. Now you really don't want to have you mage fall first(the mistake i made when fighting the high dragon) because then your kinda done. Unless your just plain destructive, the fight will probaley come down to one or two of your people. You really want to potion then because one or two hits can put them down. I hope this helps you alot!

Are you talking about The Keeper or Witherfang?

The main problem with this boss is that the dragon's lair has a bunch of fire traps that keep resetting. Now unless you're planning on having your rogue attempt to disarm all these traps during the battle - really, not a great idea, and lower-level rogues won't be able to spot the traps let alone disarm them - my suggestion is to move the fight elsewhere. You can try to disarm them before the battle, but I don't believe it's possible to get them all without triggering the fight.

The ideal place is the huge open space right at the beginning of the ruins, at the bottom of the steps (I call this the 'arena'). Set up your long range troops around the edge of the area, and send your bait/tank in to trigger the dragon. Once the dragon attacks, run like mad back to the arena, use Taunt/Threaten/whatever to keep the dragon attacking your bait (Alistair will usually be best, Shale in Stoneheart mode is also very handy), and keep shooting away. Use mages to heal/buff your bait/tank as required.

For added effect, line the route from the dragon's lair back to the arena with traps of your own. Even a bunch of basic shrapnel traps will do some decent damage.