I can now consistently beat the Adra Dragon on Normal with my party: PC (Druid), Eder, Durance, Kana, Sagani, Hiravius. And by "consistently" I mean: at least 3 times in a row. This is my first party, it's suboptimal, and far from minmaxed.
The best strategy goes as follows:
1. Eder wears heavy armor and large shield. (In my case his talents are not very defensive: Vigorous Defense, Armored Grace, Snake Reflexes, Adventurer, Interrupting Blows, Bloody Slaughter. He wears Shod in Faith, boots which grant Consecrated Ground when damaged. He wears an armor granting Second Chance (the heavy one from Endless Paths, or another heavy with the green Second Chance ring)
2. Eder goes along the bottom edge of the area. This way, not only he faces the dragon alone at first (so party is not damaged by Wing Beat and Breath), but he makes the dragon face away from the party. Subsequent breaths won't hit my party.
3. Once combat starts, use Vigorous Defense. Then, if his health is still above 75%, drink the potion granting +25 fortitude +20 reflex. If wounded below 75%, drink a potion of Major Endurance. 5 potions are enough for the entire battle and there will be some left.
The rest of the party doesn't try to help or buff Eder. It's not worth the danger and he can handle it. Even if he goes down, Second Chance will help and the dragon will be out of breath for quite a while. Either way he will buy a lot of time.
Everyone else wears medium or light armor. Meanwhile the rest of the party uses some buffs and approaches the dragon from the other side:
* Durance casts Prayer Against Fear to counter the -20 penalty of Terrified. Then casts Champion's Boon on Sagani (+10 Might, +10 Perception, 5 DR penetration). Then he stays far from danger, picking stray xaurips etc and casting any buffs you fancy.
* Kana uses Scroll of Valor, preceded by Scroll of Untamed Wild so that everyone's potions last longer.
* Sagani drinks Potion of Deletrious Something (1.5x attack speed) and shoots either the dragon or some minor monsters, if available. Save the fox for minions, keep away from dragon.
* Both druids cast Returning Storm. Then they position themselves just close enough to keep zapping the dragon, but wait a few seconds until dragon uses area attacks on Eder.
Once everyone joins the battle,
* Kana takes uses a stack of 5 scrolls of maelstrom on the dragon. Taking out some monsters first may be better, especially if they're summoners. This has a chance of benefiting from a debuff on the dragon. Kana's main objective is to keep casting Urdel and Gurdel next to dragon. Maelstrom in the remaining time.
* Druids cast the following spells on the dragon, in no particular order: Blizzard, Solar Ray, Nature's Mark (rarely works). If xaurips show up around the dragon, also try Hailstorm, Insect Swarm. Almost all listed spells attack Reflex, the weakest defense of the dragon, and preferably cold. Try to alternate spells, because many of these attacks also debuff. I tried casting the maw spell (piercing damage + prone) but couldn't hit reliably, so I mostly avoid it. When Returning Storm ends, recast it. It does a great job interrupting and damaging the dragon. It's not ideal on minions, so if there are several around you cast Relentless Storm instead. Hailstorm is not so great without minions because it's instant, something that hampers the dragon for a while is good.
Sometimes accident happen and someone gets knocked out. But Kana carries a scroll of revival, and Durance can revive. My Kana also has the revive invocation. You also have the sacrificial fox. In dire circumstances it can try to knock the dragon out.
Why both large shield and heavy armor on Eder ? Isn't heavy armor a waste on breath and wing beat ? It is. But the dragon also hits in melee for 60, and shaving 20 from that is good. Also, watch out for falling pillars.