Playing on PotD, without a priest:
N. 1: Alpine Dragon
N. 2: Lengrath
N. 3: Concelhaut
I managed to finish Lengrath with relative ease at 14 by equipping my party with anti beast equipment, engaging the adds first and having my rogue take Lengrath out ASAP. Even so, I had to wait until level sixteen before I could take the Alpine dragon out. I have no idea how other people are able to finish the alpine dragon at level eleven or even lower..
There are some keys to successful dragon hunting. (Some might say cheese.)
1. Learn the dragon hunting skill from Falanroed as part of the Adra Dragon "hunt". It reduces any dragon's defenses for a shortish but useful period of time.
2. Paralyze spell scrolls. It may seem cheesy, but dragons are so incredibly powerful with their breath attacks that can just about nuke an entire party, I'd call it the proverbial case of "fighting fire with fire".
2a. If you have a high enough level wizzy, Gaze of the Adragan for more paralyzing. A paralyzed dragon hurts no one.
3. Watch the dragon's DR's and immunities closely and use weapons, spells, chants, whatever that target the weak areas.
Paralyze scrolls didn't work well for me even before the dragons were buffed, but I agree that, sadly, immobilizing them is the (only) way to go. I just don't know how to deal with 300 non-crit semi-ranged butt attacks otherwise. The tools now have to be tailored somewhat to the dragon, because of the immunities, but not the process itself.
I find it, that despite them being resistant to prone, Concelhaut's Crushing Doom is still invaluable in these encounters, since it gives you multiple prone checks for the price of one spell (and damage's a nice bonus too). Dexterity's usually one of the lowest resistances on the dragons to begin with. The downside is, that you have to have already beaten Concelhaut to get it.
Stun weapons are a good choice too. Stunned affliction also suspends prone lengthening the effect.
Apparently, some dragons (the Alpine dragon, maybe) are immune to paralyze spells. But some are vulnerable to confusion. Some are vulnerable to prone attacks. All of this falls under point #3 about paying attention to a dragon's DRs and immunities.
One thing about dragons who aren't immune to confusion spells. Getting a dragon confused isn't going to allow you to attack and damage it freely, like you could to a paralyzed target. The moment you strike a confused target, the confusion effect goes away. The benefits I can see of getting a dragon confused are:
1) Any adds the dragon has may attack him, and he may attack his adds. But watch out, I don't know if a dragon's breath weapon is foe-only or not. And if he were to make a breath (or wing buffet) attack on his adds, if not foe-only, those attacks may hit you as well.
2) Having a dragon confused may give you time to re-position or cast buffs or healing, etc on team mates. Note that this may not quite be the case in the battle where there are two dragons. And in any case, whether one or two dragons, the other enemies may choose to continue attacking you.
And yes, stunning weapons may be nice, as long as that particular dragon isn't immune to being stunned.
One of the things I've learned in playing this (and the older IE games) is that sometimes one should look at your first attempt to any given battle (the tough ones, in particular) can be seen as gathering intel on the enemy. Oh, at times you may win on your first attempt. But even if you don't, you've learned what their strengths and weaknesses are, if you were paying attention to your enemies' DR's and immunities. And then when you give it a second try, you should be more ready.
On this same line of thinking, something else I've learned is that sometimes the class mix of characters you have in your party may not be effective against certain enemies or groups of enemies. I found fighting the monks in the Abbey very difficult with the first party I took into the Abbey (Rogue PC, Eder, Pallegina, Maneha, Durance, and a merc wizzy). So, I retreated from the Abbey and went all the way back to Caed Nua and replaced Maneha, the barbarian, with Kana, a chanter, and replaced the merc wizzy with GM, a cipher. And when I went back to the Abbey, things got a LOT easier. Kana was more durable and his chant that does flame damage to all enemies nearby was quite useful. And GM is more durable than a wizzy, and between her spells and her Stormcaller bow, she was very effective.
So it seems to me that the lesson is that there's nothing wrong with changing up the roster of your party if it will help you more easily deal with certain enemies.