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Let me think. Great Wyrm White Dragon has a huge SR and AC. The melees are going to have a hardtime hitting it.
Probably going to need to fully buff the melees with Bull's Strength (If they don't already have better strength items.), Greater Magic Weapon (If they dont' already have better magic weapons), Haste, and Blur/Displacement. (They will need the miss chance.) Probably want to have Jamison polymorph the melees into Stone Giants. (Give them a huge armor boost, higher natural strength, and reach so they can close on the creature with 5 foot steps.)
The other buffs you probably want to cast for or on your melees are as follows:
1. Stone Skin: 10 DR will greatly enhance their survivability and Shield of Faith (+4 deflection bonus, unless you have rings of protection that are better.)
2. Spell Immunity: Pick a couple of spells Accessiwal is known to cast under fourth level.
3. Energy Immunity.
4. Prayer
5. Bless
6. Load them up with healing potions if they don't already have them. Probably going to be using the haste actions to quaff them.
7. Fly spells (Very Important as they will be taking a 5 foot step probably as the flying creature passes over. Dragon has 15 foot reach. Giant has 10 foot reach. They can take a 5 foot step and an attack as a ready action to strike it as it flies over.)
8. Make sure both your melees are armed with one or two shortswords with greater magic weapon cast on them. Just in case the dragon decides to grapple with its bite. (Remember, -20 on its grapple checks unless it gives up its other attacks just to grapple. You should be able to avoid claw grapples because you are large.)
9. Might want to pick up a shield potion or two to get their A.C. up even more. (A.C. Bonuses beyond normal Armor should already look like the following: -1 size (Giant Form) +11 Natural (Giant Form) +4 Deflection (Shield of Faith) +4 Dodge (Haste) = +18 before adding in armor and enhancement bonuses. Still not much for a Great Wyrm, but better than nothing.)
Variable Option: Equip Grumbar and Shade with Longspears with Greater Magic Weapon. This will even out the reach between them and the dragon in Giant Form.
(Attack roll after all the buffs should be fairly Good: +10 (Str) +14 (Base Attack) -1 (Size) +4 (Magic Weapon) +1 Morale (Bless) +1 Luck (Prayer) = +29 Attack Bonus (Still need roughly a twelve or better to hit the dragon. Need to ensure that the dragon is fully debuffed.)
They are going to have to take alot of ready actions to take advantage of their reach, but the dragons AC is so high, they mine as well.
Now you need to set up yourself and the other casters in a safe place relatively close to the melees. The following is assuming you are fighting in a mountainous region:
1. Create a stone sanctuary using disintegrate spells preferably near or slightly inside the dragon lair. You can either create a tunnel from the outside or hope the dragon is deep enough in its lair that it does not notice you near the entrance. Pick a spot where the melees can actually fight while still in sight.
2. The melees are going to have to come to you for healing, because you really don't want to be in melee range with the dragon. Make sure you are standing out ot its reach in the cave or have anti-life shell up.
3. Have your arcane casters ready to seal off the melees with a wall of force if they are low on health so they can run back into your stone sanctuary. Then use another wall of force to seal it off while you heal up the melee for more action.
4. The dragon may also use obscuring mist or another type spell, so have a spell ready such as Gust of Wind ready to get rid of it.
5. Be ready to teleport, preferably with the entire group.
Your best shot at killing the dragons is to use melee damage and an occassional spell attack. The entire group should be doing their best to support the melees to prevent them from dying while they hack away at the dragon.
They may be grappled as well that is why a fly spell is important just in case the dragon flings them or takes them to air. If they are grappled, they will most likely have to just stab the dragon while they live unless someone can fly to them and dimension door or teleport them out.
You may not initially kill the dragon. Even if you don't, you can set yourself up better this time.
Go into the adventure with the goal of stealing a good portion of the dragons horde. The dragon will want its horde back and if you do enough damage, it will probably take a while for the dragon to heal. This will give you time to use its horde to draw it to a location of your choosing where you can set up a huge dragon trap such as a carved out ceiling filled with alchemist fire that only takes a powerful sonic or disintegration attack to unleash on the dragon.
You can purchase a huge amount of alchemist fire. The wizards can help carve out the huge cavernous hole that you will fill with alchemist fire. They can cast veil and other counterscrying spells to make sure the dragon does not pick up on your plan. Then you can set up your knights of Wee Jas in a position to pour all that alchemist fire into the huge hole just before you lure the Great White Wyrm to a fiery death in some cave. Don't forget to cast dispel on the dragon. Clear out all those pesky anti-fire buffs.
Let me know what you think of this plan so far.