How many level 9 PCs needed to be able to kill an adult red dragon?

[Aimeryan]

I don't think you are using the fly speed (+Wing Attack LA speed) and breath range of the dragon that well - given that this is the speciality of a dragon it is not surprising it would go down otherwise like any other chump.

If used well:

• The casters are going to be out of range for a lot of spells (including Fireball) and many damage spells are going to be saved for none/half if they are somehow in range (even Dex Save is +6 - other than Int it only goes up from there).
• Ranged attacks are going to have disadvantage if in range at all (this being the Strength Javelin Fighter, now with a Longbow?, and the Throwing Axe Barb).
• Melee attacks are obviously out, even if they can fly (unless fast).
• The breath attack should knock out some of the party that it hits (not all the damage will roll average, so just a little higher will get some of them). For any targets where the damage rolls average (or low for the lower hp members), or if they save (which is difficult: Dex Saving Throw DC 21 means most of the party are going to have to roll a 19+ to take half damage, with even the Rogue still failing most of the time at level 9) or have resistance (Absorb Elements most likely) a second breath attack will cinch the deal. If it really wants to prioritise a target, it can ALSO do up to three Tail Attacks with Legendary Actions (for a total of around 50 damage, with a VERY high chance to hit).

The Rogue shouldn't be able to keep up, even with a flying race. Aasimar Protector fly speed is 30ft, Cunning Action Dash gets this up to 60ft - Adult Red Dragon fly speed is 80ft, plus Wing Attack LA for another 40ft. If the Rogue is Hasted then using the double speed and the extra dash (so 180ft speed) they can keep up... until Haste is lost which should be pretty easy for the dragon to manage with a nice breath on the caster. Actually, since the Dragon is using it's action for Breath only, they could just Dash as well (so, 200ft) when waiting for the recharge, keeping ahead of the Rogue then easily. Also, it would be very stupid for the Rogue to follow the dragon out of range of their allies and fight the dragon solo, which is basically what such a chase would end up doing.

The dragon can largely ignore the Barb, Fighter, and Rogue if it circles and just comes in range for breath attacks against the party - or if the Rogue is Hasted, kill the solo Rogue, or kill/disrupt the caster. The casters are likely toast if they don't Absorb Elements, and likely toast the next breath even if they do, meanwhile most of their spells will be out of range, low damage, saved, or legendary saved.


What the party can do against this:

• Spam Earthbind, maybe? It has a 300ft range, so likely have two rounds before the dragon can outrange it after using it's breath, then you have to wait for the second breath. If enough of them can cast the 2nd level spell they might get lucky; the Dragon has +8 to Str Saves, and it has 3 Legendary Saves... presuming the first breath doesn't kill the casters. The only (relatively) weak save the Dragon has is Int (+3); there are Int Saving Throw spells potentially available at these levels, however, they are all short ranged so unlikely to get any casts of unless the caster(s) win initiative and start near enough - if the casters could then force out the Legendary Saves using these they may be able to follow up with an Earthbind before the dragon gets out of range.
• Hide in some tunnels and hope it follows?