I'm not sure what you want my help with.

@BESW I think that the level 9 solo, taken out of a favorable environment, is not really the hard fight the players expect (they avoided confrontation because they feared the dragon) and I wanted to craft some cultist NPCs and some terrain features that can turn the dragon encounter into something interesting.

Hmm. Okay, the cultists should synergise with the dragon and the terrain should be negotiable so that both sides can benefit from it or be hindered by it.

The party is made of a pacifist cleric, who mainly heals and debuffs; a charge barbarian; a mind-control wizard (moving enemies and having them attack each other, which is going to tyurn badly if he gets the dragon, I guess); a pyromancer elementalist and a slow/prone warden.

First thought: As a minor action, a cultist can transfer a debuff from a nearby ally to himself.

That lets them keep the dragon in play despite the party likely stacking debuffs onto it, but doesn't just negate the powers the PCs are using.

I'd make a lot of difficult, covering, and blocking terrain: debris and half-fallen buildings make for a battlefield where ground-bound characters duck and cover a lot, and can be easily pinned down.

The dragon, with flight, can attack everyone with impunity--but it's also visible to everyone.

I'd make the cultists leader types: they hunker down behind cover and keep the dragon a big threat for as long as possible. This forces interesting choices about who to target and how.

I had the idea of having cultists heal the dragon if not dealt with

I'm uploading the statblock for the dragon, it's gonna be a temporary link for copyright issues.

I think the Instinctive Dark Assault power would be hampered by lots of difficult terrain.

But the dragon is razing down houses so rubble and half-demolished walls are probably going to be part of the scene.

....heheheh. Okay, so instead of debris creating difficult terrain, make it ice.

The dragon's been freezing the town as he rips it apart.

The dragon's icewalk will let it ignore the effects of the ice.

Also, add this ice effect: when a creature is pushed, pulled, or slid more than 1 square, it also falls prone at the end of the forced movement.

I will also point out that the dragon can fly, and for the first part of the session it should probably be using Dark Assault by flying down to the ground, moving through enemies' squares, then going airborne again at the end.

When it's bloodied, and has a massive boost to its chance to do critical hits, then it should get into melee, lashing out with claw and dragon's fury to make as many attacks as possible.

Before it's bloodied, play it canny and ranged.

@BESW This was how I figured it. Even if it is a white dragon, it's not that stupid. It will play to its strenghts. Maybe there's some minions at ground level to keep the melees busy while the ranged guys try to get the dragon in range or to downright damage it. And the dragon is not going to spare them with the occasional breath weapon or tail swap

I'm fond of kobolds for dragon minions, flavouring them according to the dragon's themes.

So... kobolds with icewalk and a little bit of slowing debuff or cold damage, maybe.

This time I already have a human dragon cult ready. They've been introduced as the owner of an estate and his shady guards, before the characters ascended to the planar prison. When asked for help, they downright said no.

Ah, okay.

I like giving minions lasting death-triggered effects, turning the spot they die into a permanent terrain obstacle.

And now they're pillaging and blessing Tiamat for the dragon she sent them.

How about... the spot a cultist dies [inflicts 5 cold damage to]/[slows] anyone except dragons who enters it or starts their turn in it, and expands a dragon's breath weapon recharge by 2 (to 3/4/5/6 instead of 5/6) when the dragon rolls recharge while standing on the spot.

I notice that the pyromancer won't have any particular advantage because of his fire specialty. Interesting.

No vulnerability. This thing is irking our ex 3.5e/PF player. He's a dragonborn warden and he's baffled that his race does not grant him any cold immunity or resistance.

I'm a big fan of giving solos triggered actions, so I'd be tempted to give the dragon Vulnerable 5 Fire, and then say every time it takes fire damage it inflicts half that damage in a close burst one.

Since we agreed to roleplay him as a half-dragon, the players argued he's top-half dragon and bottom-half human.

(Steam, or ice shards, or something.)

(Or just him rampaging in pain)