Just for fun, I have a
core level 20 sorcerer which we could use to try and establish if there are any weaknesses with the dragon's plans.
What is the field of battle, and what is the dragon's method of attack?
Although I don't expect any players of my group to be around, I still want to give a spoiler alert first. So... isoChron, DON'T READ ANY FURTHER!!!
Now, as requested...
The battle field is the slope of an erupting volcano of some 3000 ft height. The volcano is producing a large ash column with a sporadical ejection of lava bombs from the central crater (25 % chance per round that a 1-ft-sized bomb lands in a 90x90 ft area (i.e., size of a small battle map), chance that it may hit a creature is 1 % per 5 ft. square of the creature => large creatures will be hit more often). It becomes obvious that the volcano is rapidly increasing its activity and a major eruption is about to take place.
The slope is about 25 degrees, the surface is rocky and covered with ash. No run and no charge upwards. Each movement enforces a Balance check DC 12 (10 for the slope + 2 for the ash). Steep gullies are present here and there, making movement in straight lines difficult.
The PCs stayed in an old mining complex created by fire giants in the volcano long ago before it resumed activity. They will exit the complex at night (why see below) via a small cavern in the volcano's flank at about half-height, i.e. 1500 ft above the surrounding landscape. The entry of the cavern will collapse shortly after their exit due to the increasing volcanic activity: parts of the mine tunnels work as a vent for volcanic gasses, and hot ash and lava is blown out of the cavern at the volcano's flank (see the eruption of Mt. St. Helens 1980, and you'll understand what I mean).
The ash column blocks sun- and moonlight, the ash clouds are about 200 ft. above the surface. The ongoing eruption creates a lot of noise, imposing a -10 circumstance penalty to Listen checks.
The evil party is/was a strong cell of the Cult of the Dragon:
1 very old red dragon (supporting the cult and considering to be transformed into a dracolich)
1 necromancer 13
1 cleric 9 of Velsharoon
1 wizard level 7
4 orcs bbn 2/ftr 2 (the remaining melee force of the local cell, the rest was wiped out...mostly by the pseudodragon familiars of the PCs

)
10 gnolls of a local tribe, paid by the cult for scouting and fending off weak intruders; all ftr 1/bbn 1
The plan
The fighters are scattered behind rocks, in gullies, all at different locations. None of these fighters is closer than 40 ft to the next one (no mass-annihilation via fireballs). They will fight with longbows, and change to battle axes if pressed into melee. They are sacrificed by the cultists in order to disperse the party and hopefully deplete the spell pools of the PC casters.
The three cultists are well hidden behind an Illusory Wall 20 ft. uphill and 100 ft. left of the entry of the cavern to have a good view onto the battle field. They will try to interfere with enemy casters only when the party is about to regroup for retreat. They have cast Darkvision. Glowing lava bombs and illuminate local ptaches of the battle field, but concealment is a nasty thing still. Gnolls and orcs have darkvision
The dragon has cast Disguise Self to camouflage himself as a rock formation (heck, I've seen a dragon-like rock formation on Iceland, in Drekagil) and is located between the rocks, far uphill above the cavern (some 150 ft.).
Once the fight begins, the dragon should act... Options:
Round 1: AMF, or just fire-breath the group as long as it is packed together? Other options?
Round 2: retreat into the ash cloud, cast AMF? Or if AMF is already up: grab a nice hot lava bomb?
Round 3: drop the bomb, fly-by and snatch a caster?
The adventurers: a sub-optimized epic group (level 25), strongly under-equipped (we agreed on a campaign with less magic items), but in possession of an evil artefact related to time (Klorr's Timepiece, but modified):
2 human melee types (ftr, ftr/mnk/drd 8)
2 arcane casters, one human, one elf (mnk/wiz/enlightened fist (CL 23), rog/warmage/daggerspell mage (arcane CL 14)/favored soul 8)
1 human divine caster (mnk/clr 14/pious templar 4)
1 juvenile vampiric brass dragon (brd 4/spellfire channeler 6). Don't ask about his background, it is a very long story. His spellfire is drained due to previous actions, 6 levels left.
1 very young brass dragon, brother of the juvenile dragon. The weakest member (7 HD only), a NPC, but the group owes him a lot and should protect him.