Wind wall blocks gaseous breath weapons.
Heroes' feast grants immunity to poison, among other things, for 12 hours. It is a 6th level spell, so you would most likely need to obtain a scroll or two.
Be careful with plane shifting yourselves to the Positive Material Plane. The fast healing property can kill you very quickly if you are not prepared for it.
I would look at tactics designed to force your opponents to engage you on the ground. Taking the fight to the air is fighting on their terms.
Try scrying your opponents to get an idea of their organization and capabilities. If you can target their leader first in your battle, you may be able to throw them off balance.
Command undead on the mounts could remove a few of them, and their riders, from the battle.
Searing light is great against undead.
Target the mounts first. The mobility they provide is a large part of your problem. Also, if you destroy one while it is in the air, you will most likely damage the rider when it falls to the ground.
With enlarge person, haste, and greater invisibility your archer may be able to take down one of the wyverns before they realized what was happening.
Halt undead could be very effective against the wyverns.
Heroes' feast grants immunity to poison, among other things, for 12 hours. It is a 6th level spell, so you would most likely need to obtain a scroll or two.
Be careful with plane shifting yourselves to the Positive Material Plane. The fast healing property can kill you very quickly if you are not prepared for it.
I would look at tactics designed to force your opponents to engage you on the ground. Taking the fight to the air is fighting on their terms.
Try scrying your opponents to get an idea of their organization and capabilities. If you can target their leader first in your battle, you may be able to throw them off balance.
Command undead on the mounts could remove a few of them, and their riders, from the battle.
Searing light is great against undead.
Target the mounts first. The mobility they provide is a large part of your problem. Also, if you destroy one while it is in the air, you will most likely damage the rider when it falls to the ground.
With enlarge person, haste, and greater invisibility your archer may be able to take down one of the wyverns before they realized what was happening.
Halt undead could be very effective against the wyverns.