The fight is usually a lot easier when you have a Priest, a Druid and a Wizard (fully rested). You don't need a real tank - if you can reliably disable tough foes. 3 relatively sturdy frontliners are enough. And I think a Monk is better compared to a Fighter in general, the only thing the Fighter is better at is gaining high accuracy quickly (Disciplined Barrage) and use it with several Knockdowns - which is very helpful to occupy the dragon very early in the encounter. Monk needs more time because of wounds. He can stun right away - but with less accuracy and a lot shorter duration. But his Force of Anguish can make up for that because its prone duration is very long compared to Knockdown. If you give the Monk Veteran's Recovery there shouldn't be such a big difference between him and a Fighter in terms of sturdyness.
Relentless Storm, Chillfog, Confusion and Ninagauth's Shadowflame make short work of all mobs in a very short time. And Chillfog blinds/Relentless Storm stuns the Adra Dragon, too.
Also Wizard and Druid both have petrifying spells, which - when applied to the dragon - makes the damage dealing part so much easier: besides the monstrous defense debuff they suffer, petrified enemies take double the damage.
Druid's Charm Beast and Hold Beast Druid are excellent in this case, too.
A Chanter with Dragon Thrashed is useful to just erase the mobs without having to do much. You can run him into the enemy crowd and when he's about to go down you can withdraw him with Durance: he then becomes untouchable but will still burn and slash everything around him. His accuracy will go down a bit, but oddly enough a single one handed weapon does increase the accuracy of offensive chants such as Dragon Thrashed by +12, so he should still be able to hit mobs reliably enough.
My last fight against the Adra Dragon was with this party comp. (outfit, most used stuff):
front line:
Eder (Tidefall, Sanguine Plate, 4 Knockdowns, Charge)
Kana Rua (Edge of Reason, Argwe's Adra, The Dragon Thrashed)
Hiravias (Hearth Harvest+except. hatchet, Wayfarer's Hide, Relentless Storm, Overwhelming Wave, )
back line:
Durance (except. Arquebus, Angio's Gambeson, prayers + acc buffs)
MC Chanter (One-Eyed Molina's Spike Flinger, Sure Handed Ila + Aefyllath Ues Mith Fyr)
Aloth (Gyrd Háewanes Sténes, Rundl's Finery, Confusion, Chillfog, Ninagauth's Shadowflame)
It was very easy and the fight was over quickly. I cannot stress enough how important it is to be able to buy a bit of time at the start of the encounter (if you don't have party members who can just facetank the dragon).
I'm currently running another party: with MC Barb Firebrand, Eder dw war hammers, Devil of Caroc dw sabres, Zahua fists, Durance arquebus & Aloth rod.
It's way(!) harder to beat tough encouters on PotD - especially the ones that are a bit overleveled compared to me. I like to go to certain places too early just to pick up a few items I want to habe early.
My current setup lacks the additional oomph that a per-rest caster such as a Druid can unleash at will. A Rogue or Monk cannot make up for that at all (although I do enjoy playing them, esp. the Monk).
What's also very helpful against dragons in general is Munacra Arret, Spirit Spiral and Ring of Changing Heart on a high accuracy character. All but one dragon can be charmed or dominated. And it's easy to hit with a charm spell that has +15 accuracy bonus and is on a character who's already giot good accuracy.