An extreme example of a multi stage creature I have used is a parasitic entity from the far realms called second skin. It survives by joining with a host and using the host power in conjunction with its own.
This is a 3 stage boss, which starts as a Huge creature and reduces in size at each stage, ending up as a medium sized creature. The first stage is Second Skin joined with a Fell Troll, when the this form becomes bloodied the body of the Fell Troll cannot contain Second Skin any longer and its flesh sloughs off revealing Second Skins next form, resembling a Living Statue. When this form is reduced to 200 HPs its stone body crumbles away to reveal its final base form which is mass of black tentacles.
Second Skin Fell Troll Level 20 Solo Brute
Huge Aberrant Humanoid (phase 1) XP 14,000
Initiative +16 Senses Perception +20, darkvision
HP 1190; Bloodied 595; see Shed the Skin
Regeneration 10, if takes acid damage regeneration doesn't function next
turn
AC 34; Fortitude 36, Reflex 32, Will 34
Immune charm, disease, poison;
Resist 20 lightning, 10 radiant;
Vulnerable 15 psychic, 5 necrotic
Saving Throws +5
Speed 10
Action Points 2
Stone Tipped Claw (Standard; at-will)Reach 3; attack +23 vs. AC; 2d6+8 damage
Backhand Slam (Standard; at-will)Reach 3; attack +21 vs. Reflex; 2d8+8 damage; on hit target is pushed 4
squares and knocked prone
Ferocious Attack (Standard; at-will)Second Skin makes 2 Stone Tipped Claw and 1 Backhand Slam attack
Crunching Rush (Standard; recharge 
)Second Skin moves up to 10 squares in a straight line, this movement
provokes opportunity attacks as normal; when any part of this movement
would take him into an enemies space he may make the following attack;
attack +20 vs. Fortitude; 1d10+8 damage; on hit Second Skin may either
grab target and carry them along or slide them 2 squares and knock them
prone, Second Skin may only grab and carry 2 targets, at the end of the
movement Second Skin makes an attack against each grabbed target;
attack +20 vs. Reflex; 2d8+8 damage; on hit target is released from the
grab, prone and dazed (save ends); on miss target is released from the
grab
Eye Beam Strafe (Standard; encounter) ♦ radiantClose blast 5; attack +21 vs. Reflex; 4d12+7 radiant damage; on hit
target takes a -4 penalty to attacks (save ends); on miss half damage
Bellow of Revenge (Standard; encounter) ♦ fearBurst 10; attack +21 vs. Will; on hit target is stunned until the end of
their next turn, after effect target takes a -2 penalty to attack (save
ends)
Shed the Skin (Immediate Reaction) ♦ when first bloodied
When first bloodied Second Skin sheds the Fell Troll revealing its ultimate
form, see separate stat block
Threatening Reach
Second Skin has threatening reach (3) with its Stone Tipped Claw attack
Alignment Evil Languages deep speech, common, giant
Skills Athletics +23, Dungeoneering +20, Endurance +24
Str 26 (+18) Dex 22 (+16) Wis 20 (+15)
Con 28 (+19) Int 18 (+14) Cha 24 (+17)
-----------------------------------------------------------------------
Second Skin Level 20 Solo Controller
Large Aberrant Humanoid (phase 2) XP 0
Initiative +16 Senses Perception +20, darkvision
HP 595; Bloodied 297; at 200 HPs see Shed the Stone
AC 36; Fortitude 34, Reflex 33, Will 34
Immune charm, disease, poison;
Resist 20 lightning, 10 radiant;
Vulnerable 15 thunder, 5 necrotic
Saving Throws +5
Speed 6
Action Points 1
Stone Punch (Standard; at-will)Reach 2; attack +23 vs. AC; 3d6+7 damage
Lightning Charged Punch (Standard; at-will) ♦ lightningReach 2; attack +23 vs. AC; 2d8+7 damage; on hit target takes 10
ongoing lightning damage (save ends)
Double Punch (Standard; at-will)Second Skin makes a Stone Punch and a Lightning Charged Punch attack
Ultimate Control (Standard; recharge ) ♦ psychicLine of sight; does not provoke opportunity attacks; attack +24 vs. Will;
on hit target is dominated (2 saves ends), note Brand gains a +5 bonus to
Will defence vs. this attack
Lightning Arc (Standard; recharge ) ♦ lightningClose blast 3; attack +24 vs. Reflex; 4d8+7 lightning damage; on hit
target is dazed (save ends)
Eye Beams Strafe (Standard; encounter) ♦ radiantClose blast 5; attack +24 vs. Reflex; 4d12+7 radiant damage; on hit
target takes a -4 penalty to attacks (save ends); on miss half damage
Share My Pain (Immediate Reaction; recharge
) ♦ psychic, when damaged
When damaged Second Skin may psychically cause damage to a nearby
creature and an immediate reaction; range 5; attack +20 vs. Will; on hit
target takes an amount of psychic damage equal to half the total that
Second Skin took from the attack
Shed the Stone (Immediate Reaction) ♦ when reduced to 200 HPs
When reduced to 200 HPs Second Skins stone flesh falls away revealing its
ultimate base form, see separate stat block
Alignment Evil Languages deep speech, common
Skills Dungeoneering +20, Endurance +20, Insight +20
Str 24 (+17) Dex 22 (+16) Wis 20 (+15)
Con 20 (+15) Int 26 (+18) Cha 24 (+17)
-----------------------------------------------------------------------
Second Skin (Base Form) Level 20 Solo Skirmisher
Medium Aberrant Beast (phase 3) XP 0
Initiative +20 Senses Perception +20, darkvision
Aura of Paranoia (fear, psychic) aura 5; Second Skin and all creatures
inside aura count as both allies and enemies, and take all benefits and
disadvantages from this (can provide flanking, take both ally and enemy
effects from powers and abilities from effects or creatures inside aura)
HP 200; Bloodied 100; see Refusal to Die
Regeneration 10, regenerate does not function next turn if Second Skin
takes thunder damage
AC 36; Fortitude 31, Reflex 35, Will 35
Immune charm, disease, poison;
Resist 10 radiant, 10 fire
Saving Throws +5
Speed 6, teleport 3
Action Points 1
Tentacle Rake (Standard; at-will)Attack +25 vs. AC; 2d6+8 damage; on hit target takes 5 ongoing damage
(save ends)
Double Rake (Standard; at-will)Second Skin makes 2 Tentacle Rake attacks; if both attacks hit the same
target they takes 10 ongoing damage (save ends)
Psychic Tornado (Standard; recharge ) ♦ psychicBurst 3; attack +21 vs. Will; 3d8+8 psychic damage; on hit target is
pushed 6 squares and knocked prone
Mass Domination (Standard; encounter) ♦ psychicBurst 5; attack +20 vs. Will; 1d6+8 psychic damage; on hit target is
dominated (save ends); Brand gets a +5 bonus to Will defence against this
attack
Refusal to Die (Immediate Reaction) ♦ necrotic, when first reduced to 0 HPs
Burst 5; attack +22 vs. Fortitude; 2d6+8 necrotic damage; on hit Second
Skin regains a number HPs equal to the damage taken by target
Combat Advantage
Second Skin deals 2d6 extra damage with melee attacks when it has
combat advantage
Alignment Evil Languages deep speech, telepathy 10
Skills Dungeoneering +20
Str 20 (+15) Dex 26 (+18) Wis 20 (+15)
Con 18 (+14) Int 22 (+16) Cha 26 (+18)
