Thanks for the great suggestions I've received thus far! Taking the advice into account, I've statted up Aggrondaxilis, my elder green wyrm. Bear in mind that my PCs are 20th level.
Party looks like this:
Narissys - Eladrin Tempest Fighter (AC 37, routinely boosted)
Will Straum - Human Cleric (melee focused, good group buffs)
Jaski - Human Rogue (highly mobile, very good single-target damage)
Brandon von Redding - Human Ranger (highly mobile, very good single-target damage)
Virgo Blaze - Human Sorcerer (highly mobile, great multi-target damage, good single-target damage)
Phase One
His first phase is a playful, cat and mouse snatch-and-grab keep-away type of combatant, where he'll try and snatch one PC, carry them off, slow the others with his breath, and eat the lone target. His main tools here are his incredible speed, his flyby attack (which as written allows him to snap someone up and carry them off), and his swat (to prevent someone interfering with his meal).
Aggrondaxilis the Playful --
Level 23 Solo Controller
Huge natural magical beast (green dragon) -- XP 25,500
HP 360 --
Initiative +18
AC 39
F 34
R 36
W 35 --
Perception +22
Speed 10 (forest walk), fly 14 -- Darkvision
Traits
Action Recovery
Whenever the dragon ends its turn, any dazing, stunning, or dominating effect on it ends.
Instinctive Flyby
On an initiative of 10 + its initiative check, Aggrondaxilis can use a free action to use flyby attack. During the movement from flyby attack, he gains a +4 bonus to all defenses against opportunity attacks. If he cannot use a free action to make this attack due to a dominating or stunning effect, then that effect ends instead of the dragon making the attack.
Standard Actions
Bite (poison) * At-Will
Attack: Melee 3 (one creature); +28 vs. AC
Hit: 3d10 + 18 damage, and the target is grabbed. The target takes ongoing 15 poison damage until they escape the grab. When the dragon moves, the target moves with him.
Miss: 10 poison damage.
Breath Weapon (poison) * Recharge [5] [6]
Attack: Close blast 5 (enemies in the blast); +26 vs. Fortitude
Hit: 2d12 + 16 poison damage, and the target is slowed and takes ongoing 20 poison damage (save ends both).
Aftereffect: The target is slowed (save ends).
Flyby Attack * Recharge [5] [6]
Effect: Aggrondaxilis flies up to 10 squares and uses bite or breath weapon (if the power is recharged) at any point during the move.
Minor Actions
Luring Glare (charm) * At-Will
Attack: Close blast 10 (one creature in the blast); +22 vs. Will
Hit: Aggrondaxilis slides the target up to 3 squares.
Triggered Actions
Reflexive Swallow * At-Will
Trigger: Aggrondaxilis reduces a creature to 0 hit points with his bite.
Effect (free action): Aggrondaxilis swallows the target. While swallowed, the target has line of sight only to Aggrondaxilis, and no creature has line of sight to the target. The target takes 15 poison and acid damage at the start of each turn until they are no longer swallowed.
Swat * At-Will
Trigger: An enemy attacks Aggrondaxilis while he has a target grabbed.
Effect (Immediate Interrupt): The dragon makes the following attack against the triggering creature:
Attack: Melee 3 (one creature); +28 vs. AC
Hit: 3d8 + 16 damage, and the target takes a -4 to the attack roll. Following the triggering attack, Aggrondaxilis slides the target up to 4 squares.
Phase Two
Once Aggrondaxilis sees the PCs as a genuine threat, he takes things more seriously. He brings his claws into play, and he stops the grabbing game and tries to inflict some damage. His bite is used more to knock people off balance, creating opportunities for combat advantage. He is focused on using his breath weapon as much as possible.
He crits more often, attacks more, he doesn't move around as much, but his breath hits much harder and more frequently.
Aggrondaxilis the Fierce --
Level 23 Solo Controller
Huge natural magical beast (green dragon) -- XP 25,500
HP 360 --
Initiative +18
AC 39
F 34
R 36
W 35 --
Perception +22
Speed 10 (forest walk), fly 14 -- Darkvision
Traits
Action Recovery
Whenever the dragon ends its turn, any dazing, stunning, or dominating effect on it ends.
Fearsome Assault
On an initiative of 10 + its initiative check, the dragon can use a free action to use claw. If the dragon cannot use a free action to make this attack due to a dominating or stunning effect, then that effect ends instead of the dragon making the attack.
No Longer Playing
Aggrondaxilis scores a critical hit on a roll of 19 or 20.
Standard Actions
Bite (poison) * At-Will
Attack: Melee 3 (one creature); +28 vs. AC
Hit: 3d10 + 18 damage, and 15 poison damage.
Effect: The target grants combat advantage (save ends).
Claw * At-Will
Attack: Melee 3 (one or two creatures); +28 vs. AC. If the dragon targets only one creature, it can make this attack twice against that creature
Hit: 3d8 + 16 damage, and the dragon shifts up to 2 squares.
Breath Weapon (poison) * Recharge [4] [5] [6]
Attack: Close blast 5 (enemies in the blast); +26 vs. Fortitude
Hit: 3d10 + 16 poison damage, and the target takes ongoing 20 poison damage (save ends).
Aftereffect: The target takes ongoing 10 poison damage (save ends).
Flyby Attack * Recharge [5] [6]
Effect: Aggrondaxilis flies up to 10 squares and uses bite or breath weapon (if the power is recharged) at any point during the move.
Minor Actions
Luring Glare (charm) * At-Will
Attack: Close blast 10 (one creature in the blast); +22 vs. Will
Hit: Aggrondaxilis slides the target up to 3 squares.
Triggered Actions
Master Of Illusion (illusion) * Recharge [4] [5] [6]
Trigger: An enemy attacks Aggrondaxilis.
Effect (Immediate Interrupt): The enemy takes a -4 penalty to the attack roll.
Special: An enemy may make a DC 30 Arcana check as a minor action to exhaust this power.
Phase Three
At this point, the dragon is enraged. These PCs have become a mortal threat. He smashes the floor of his lair, and the hard-packed earth drops away; the PCs and the dragon fall into a lower cavern. Throughout this stage of the fight, heavier pieces of the hoard (art objects, weapons and armor) will rain down in random places across the battlefield.
Aggrondaxilis will be hitting harder. His tail is lashing about, dealing damage to anyone nearby. And he is bringing the brute power of his jaws to bear in a big way. He's exhausted much of his breath weapon, however, so his natural poison is greatly diminished. But what remains is a powerful stun, which gives him a fighting chance.
Aggrondaxilis the Enraged --
Level 23 Solo Controller
Huge natural magical beast (green dragon) -- XP 25,500
HP 360 --
Initiative +18
AC 39
F 34
R 36
W 35 --
Perception +22
Speed 10 (forest walk), fly 14 -- Darkvision
Traits
Action Recovery
Whenever the dragon ends its turn, any dazing, stunning, or dominating effect on it ends.
Furious Devouring
On an initiative of 10 + its initiative check, the dragon can use a free action to use bite. If the dragon cannot use a free action to make this attack due to a dominating or stunning effect, then that effect ends instead of the dragon making the attack.
Lashing Tail * Aura 3
Any creature who enters or begins their turn in the aura takes 10 damage (unless they are grabbed by Aggrondaxilis).
Fury Of The Green Dragon
Aggrondaxilis may score a critical hit on a roll of 18-20.
Standard Actions
Bite (poison) * At-Will
Attack: Melee 3 (one creature); +28 vs. AC
Hit: 2d10 + 25 damage, and the target is grabbed and pulled adjacent to the dragon. The target takes ongoing 10 poison damage until they escape the grab. When the dragon moves, the target moves with him.
Miss: 15 poison damage.
Claw * At-Will
Attack: Melee 3 (one or two creatures); +28 vs. AC. If the dragon targets only one creature, it can make this attack twice against that creature
Hit: 3d8 + 16 damage, and the dragon shifts up to 2 squares.
Savage Desperation * Recharge [6]
Aggrondaxilis uses bite and claw.
Breath Weapon (poison) * Recharge [6]
Attack: Close blast 5 (enemies in the blast); +26 vs. Fortitude
Hit: 3d10 + 16 poison damage, and the target is stunned and takes ongoing 10 poison damage (save ends both).
Aftereffect: The target is dazed (save ends).
Minor Actions
Chew * At-Will
Attack: Melee 3 (one grabbed target); +26 vs. Fortitude
Hit: 2d10 + 16 damage.
Triggered Actions
Reflexive Swallow * At-Will
Trigger: Aggrondaxilis reduces a creature to 0 hit points with his bite.
Effect (free action): Aggrondaxilis swallows the target. While swallowed, the target has line of sight only to Aggrondaxilis, and no creature has line of sight to the target. The target takes 15 poison and acid damage at the start of each turn until they are no longer swallowed.
