Dr. Midnight,
After consulting the Monster Manual I don't see that your players have much of a chance. They are putting the cart before the horse in this situation. It would be best if they teleported away and hid as opposed to committing mass suicide. Dragons are at the high end of the CR scale and possibly 1 or 2 CRs too low, especially at the Great Wyrm age category. So what we have here is a level 14 party taking on about a CR 22 creature, I really feel for your players. I wouldn't expect anyone to make it out alive, or undead. The only exception is Vek and that's only if he reforms before Accesiwail finds his phylactery.
But if you want some advice, here goes nothing (by the way the ice golems were a nice touch).
Concerning Accesiwail's spell selection. I would recommend haste, improved invisibility, protection from elements, dispell, greater dispelling, True strike, disentigrate, and lesser planer binding.
Prep with haste, Improved invisibility, protection from elements, bull's strength, and eagle's splendor (at least this much).
Use lesser planar binding to get some tundra shamblers (about 6-8 should work perfectly). Hide them in niches in the walls and floor of the main battle room (You can find them in Manual of the Planes in the template section). They can grapple the spell casters and then constrict them.
First action (because You'll see them long before they see you) hit the Melee fighters with Greater Dispelling.
I don't know what feats you chose for Accesiwail but spell focus Transmutaion is always good as is Spell Penetration and Power Attack and cleave are almost standard.
Then cast True Strike and disentigrate Vek. The Save should be difficult for him to make at best
Heighten Spell Hold person combo would be exceptionally deadly agianst anyone with a low will save.
Just some random thoughts, use them if you want, but really I don't think Accesiwail will have any problems wiping the floor with the party.
After consulting the Monster Manual I don't see that your players have much of a chance. They are putting the cart before the horse in this situation. It would be best if they teleported away and hid as opposed to committing mass suicide. Dragons are at the high end of the CR scale and possibly 1 or 2 CRs too low, especially at the Great Wyrm age category. So what we have here is a level 14 party taking on about a CR 22 creature, I really feel for your players. I wouldn't expect anyone to make it out alive, or undead. The only exception is Vek and that's only if he reforms before Accesiwail finds his phylactery.
But if you want some advice, here goes nothing (by the way the ice golems were a nice touch).
Concerning Accesiwail's spell selection. I would recommend haste, improved invisibility, protection from elements, dispell, greater dispelling, True strike, disentigrate, and lesser planer binding.
Prep with haste, Improved invisibility, protection from elements, bull's strength, and eagle's splendor (at least this much).
Use lesser planar binding to get some tundra shamblers (about 6-8 should work perfectly). Hide them in niches in the walls and floor of the main battle room (You can find them in Manual of the Planes in the template section). They can grapple the spell casters and then constrict them.
First action (because You'll see them long before they see you) hit the Melee fighters with Greater Dispelling.
I don't know what feats you chose for Accesiwail but spell focus Transmutaion is always good as is Spell Penetration and Power Attack and cleave are almost standard.
Then cast True Strike and disentigrate Vek. The Save should be difficult for him to make at best
Heighten Spell Hold person combo would be exceptionally deadly agianst anyone with a low will save.
Just some random thoughts, use them if you want, but really I don't think Accesiwail will have any problems wiping the floor with the party.