I was thinking this kind of question would be closed due to it would be "opinion-based", but turns out there're tons of similar questions exist (and answered) already. So I am going to give it a try.
Long story short. I am trying to write (or draft?) a DND 3.5e campaign based on the story of Madoka Magica, and I come up with something like this. I wonder if that would be sufficient for a CR 18 monster (or EL 18 encounter) (since the minions seem to be pretty weak). Hopefully it won't go over the limit of a question. (Unfortunately I have to remove some parts.)
Walpurgisnacht (Last Boss)
Colossal Aberration [witch]
HD: 25d8 +250 [constitution] (362/362 hp) 20 temporary hp
Initiative: +5 (+1 Dex, +4 feat)
Speed: (upside-down) fly 30 ft. (Clumsy) (reversed) fly 150 ft. (Clumsy)
Armor Class: 25 (+1 Dex, +22 natural, -8 size), touch 3, flat-footed 24
BAB/GRP: +18/+50 (+10 Str, +16 Size, +6 Profane)
Attack: Whip of Familiar +17 (+18 [base] +1 [dex] +6 [profane] -8 [size]) (4d6+20 plus summon familiar)
Full Attack: Whip of Familiar +17 (4d6+20 plus summon familiar)
Space/Reach: 60 ft./30 ft.
Special Attacks: Building Throwing, Whip of Familiars, Flame of Helplessness
Special Qualitites: blinded, blindsight 1 mile, Flaming Aura, SR 30, Summon Familiar, Storm Aura, immunity to poison, Ring of Familiars, Helplessness, Chaos Mind, Fast Healing 20
Save: Fort +24, Ref +15, Will +26 (all +6 [profane] included)
Abilities: Str 30 Dex 12 Con 30 Int 11 Wis 22 Cha 13
Skills: (28/46) (Max Rank: 28)
Concentration +44 (all +6 [profane] included)
Feats: (9/9) Wingover, Hover, Ability Focus (Spelllike Ability: Fire Storm), Weapon Focus (Whip of Familiars), Flyby Attack, Improved Initiative, Quicken Spell-Like Ability (Fireball), Empowered Spell-Like Ability (Fire Storm)
Challenge Rating: 18
Alignment: Always chaotic evil
For the first damage Walpurgisnacht take for each round, all damage roll related to that damage will be taken as 1. Damages without a roll (e.g. any maximized spell) are not affected.
Flame of Helplessness (SU)
Anyone who takes flame damage from Walpurgisnacht would be affected by an area dispel magic (CL=10).
Whip of Familiars (SU)
Walpurgisnacht can make a ranged attack against a creature within 1 mile, dealing 4d6+20 slashing damage and summon 3 random familiars (CR 8) within a range of 20 feet at the block she is targeting. The Summon Familiar effect works regardless this attack misses or not.
Summon Familiar (SU)
As a free action, in every 5 rounds, Walpurgisnacht can randomly summon 1d6 familiars (CR 8) within the range of her Storm Aura. Walpurgisnacht would not intelligently control the familiars.
Ring of Familiars (SU)
Once per day, as a full-round action that provokes attack of opportunity, Walpurgisnacht can summon 24 advanced familiars (CR 13) around herself like a circle. All familiars will start their action immediately after the summon, which makes them share the same initiative check result with Walpurgisnacht.
All familiars summoned in this manner is considered in rage with no duration limit (+4 Str, +4 Con, -2 AC). But they can still cast spells or use spell-like abilities or perform other acts that require concentration, if any.
Building Throwing (EX)
Walpurgisnacht can throw a building at her opponents. All creatures within 300 ft range of the targeted block take 25d6 bludgeoning damage and fall prone (Ref DC 26 half and negate prone).
Storm Aura (EX)
100 miles diameter's storm. 10 miles' storm eye in the center has no special wind effect.
Control Weather can surpress Storm Aura for 2d6 rounds immediately after the casting (instead of taking 10 minutes to manifest), but the caster has to pass the caster spell check (DC=35) to successfully do so.
Flaming Aura (SU)
A 10-foot-radius spread heat aura surrounds Walpurgisnacht. Creatures subject to fire Damage, except Walpurgisnacht herself, take 2d6 points of fire Damage each round they remain within the heat Flaming. This damage occurs when their round starts.
Chaos Mind (EX)
Any creature that attempt to use mind-affecting effects on Walpurgisnacht automatically fails such an attempt and stun for one round (Will DC=18 negate). The save DC is wisdom-based.
As a full-round action that provokes attack of opportunity, Walpurgisnacht can change its side and switch to Reversed Stance, increases its flying speed from 30 ft to 150 ft.
Once per day, when Walpurgisnacht's hit point is lower than 20%, she will automatically generate a shield that can take the damage equal to 50% of her maximum hp (181) without taking any action, even if it is not her turn.
This ability works as Resilient Sphere with a radius of 50ft. from the center of her body, except it can take damage and is purely transparent.
Dispel magic, antimagic field and similar effects can surpress all (if dispel) or part (if antimagic field) of the shield and allow creatures, objects and attacks to pass by. The shield can only be surpressed for 1 round each time if dispelled (Dispel require to pass a CL check DC = 35).
Effects that would destroy force effects (e.g. disintegrate spell, rob of cancellation, rod of negation, etc.) would instead surpress the shield for 2d6 rounds and halve the current hp of the shield without dealing actual damage (the shield will be destroyed if is surpressed in this manner when the remained HP is 1).
The creatures that are inside the shield (within 50 ft. of Walpurgisnacht) can attack and interact with Walpurgisnacht normally.
Walpurgisnacht can surpress and re-activate the shield as a free action after this ability is triggered (so she can deactivate the shield, make attack, and reactivate the shield again in the same round).
Spell-Like Abilities: Caster level 25th. The save DCs are Wisdom-based.
At will -
Control Weather: Control weather (10 min cast and 10 min take effect)
Control Winds: Control the wind within 1000 fts
Gust of Wind: Blows away or knocks down smaller creatures. (DC18)
Fireball: Deals 1d6/level fire damage. (10d6 flame + Flame of Helplessness) (DC19)
Cone of Flame: Deals 1d6/level fire damage. (15d6 flane + Flame of Helplessness) (DC20) (Cone of Cold but in fire damage)
Quickened Fireball: Deals 1d6/level fire damage. (10d6 flame + Flame of Helplessness) (DC19)
Empowered Fire Storm: Deals 1d6/level fire damage. (37d6 flame + Flame of Helplessness) (DC25)
PS. Regarding the familiars, I am thinking of something with 5 Aberration HD + 5~10 class levels (becoming stronger as PCs approach closer to the boss) to give them some PVP-like experience. I wonder if that would be a good idea.
I am putting the expected PCs' brief info here:
Lv.15-16 Melee (Fighter/Paladin/Barbarian)
Lv.15-16 Ranged (Fighter/Ranger)
Lv.15-16 Divine Spellcaster (Cleric/Druid)
Lv.15-16 Arcane Spellcaster (Wizard)
This will be a core-only campaign, so only the resources on PHB, DMG and MM (feats only) would be available for the PCs. All other materials are forbidden. But they can optimize as much as they want within this limit.
The expectation of this boss is to give them a pretty tough fight, should not end in just several rounds but should be things like "deal some damage => Boss get's away with some familiars, kill or avoid those familiars and chase the Boss => deal some damage again". Since the arena will be inside the "Storm Aura", the weather condition and wind condition should be considered as important factors, and the requirement of mobility and the capability to breakthrough (rather than slaughter) the minions would be critical. There shall be no such thing of "a stable environment to stand there and keep making full-attack" in this fight.
On the other hand, the Boss itself won't be attacking PCs in most of the time, until its HP is reduced to below 70%.