The Party in my campaign is on its way to fight an Old Green Dragon. Its been "harassing the villagers, being an evil dragon, and responsible for several evil half-dragon villains in the area." So the Party is going after the Green Villain.

The party has had one run in with some of the Dragons half-dragon kin. A half-dragon/half-fog giantess (fighter) and a half-dragon/half-human sorcerer. The sorcerer was slain in the first round by a flame strike (50+ damage) and failing the Massive Damage save. The Giantess proceeded to pummel the party with her two-handed great club. The party, due to tactics, was able to survive her wrath and only end the combat "wounded". The Giantess surrendered and asked for mercy, which the Paladin and the Fighter of the party granted. She was allowed to fly home, with the corpse of her half-brother, under the condition that she no longer "attacked anyone, but defense of ones home would be permitted." She flew away from the party in a SW direction, the same direction that the Green Dragon's lair was supposed to be.

I have basically one week to detail the lair beyond my notes, and the party was planning on camping overnight, regaining their special abilities, etc. that they used against the giantess. Therefore the Dragon and kin have about 12 hours to prepare.

Thus far, most of the spells of the Dragon are defensive in nature. Mirror Image and Displacement were at the top of my list.

Besides the Sorcerer and the Giantess, the dragon also kept a Wizard son, and a Fighter/Beserker (from Quintessential Fighter), both of 10th level. The wizard is a Enchantment/Transumtation Spell Focused individual, while the Fighter is a melee machine (as usual).

The Giantess was capable of noticing several of the parties tactics. Thus far she has observed:

There is at least 2 stronger fighter types (one that has spell casting capabilities).
There is a very capable cleric.
They have 2 not so capable woodsmen assistants.
At least one Wizard who can construct walls of Force, and fireballs that delay their explosions.
She counted 6 with their party, but there could be more.

They tend to use bows to strike at larger creatures and they make use of tumbling to close on creatures with reach. While they do use bows, they are not profiecient with their use (rarely hit her when she was in melee with other creatures). The Elven Cleric can make himself invisibile, as can the Elven Wizard. She only meleed with one of the Fighters, of which he seemed rather weak, but he was capable of dealing out great damage at least once.

What the Party actually consists of:
1 Fighter/Paladin/Sorcerer
1 Fighter/Gladiator/Aristocrat/Wizard/Ranger
1 Cleric/PrC's
1 Wizard (Npc)
2 Rangers (Npcs)

Party averages about 15th level now, with only a few 14's left. The party also had a few players missing which will probably show up at the next session. Here are the ones that were missing from the standard party:

1 Fighter/Barbarian
1 Fighter/Dragonslayer
1 Cleric
1 Wizard
1 Fighter/Master Samurai
1 Rogue

Any of these individuals could show up at the next session, adding to the parties strength.

I have decided that the dragon lairs near/in a large lake within the forest, that from reports of various woodsmen the dragon modified with its own digging. What I'm trying to think of now is strategy and tactics for the Dragon and its Kin against the invaders.

I want the party to have a difficult fight. :-) What advice can ya'll give me?