Ok, so I'm updating to make it legal, and include what I promised yesterday:
EDIT: Edited the build to make sure one rod is not applied multiple times to the same spell.
EDIT: Changed the build to take care of the dual action choker trick. No the damage is even higher.
EDIT: Will remove the belt of battle/celerity trick once I'm sure those don't grant extra swift actions, I read it as extra full round actions also grant extra swift actions.
*DISCLAIMER* This build is not a super powerful build (Heck even viable, since you win anyway with shapechange), even the slightest bit of energy resistance reduces the damage vastly. *DISCLAIMER*
*IMPORTANT:* Due to me not wanting to write everything out, my Magic Missile Spells will be counted with a letter, E for those that are empowered but not maximized (most of them repeating and twinned are calculated in other categories) M for those that are both empowered and maximized. Spells with an A after are also energy admixtured 4 times (so MA or EA respectively).
Build:
[spoiler]Instant Metamagic sorcerer 6/ Incantatrix 10/ Force Missile Mage 4
Cha is maxed, everything else doesn't matter, but keep con high.
Feats:
(gets Iron Will ASAP from Otyugh hole)
(Flaw): Empower Spell
(Flaw): Knowledge Devotion
Level 1: Quicken Spell
Human: Combat Casting (has to have this for FMM)
Level 3: Twin Spell
Level 6: Arcane Thesis Magic Missile
Incantatrix 3: Repeat Spell
Level 9: Practical Metamagic Twin Spell
Incantatrix 7: Maximize spell
Level 12: Easy Metamagic Repeat Spell
Incantatrix 10: Easy Metamagic Twin Spell
Level 15: Arcane Thesis Greater Arcane Fusion
Level 18: Easy Metamagic Quicken[/spoiler]
Equipment: (760,000 gold)
[spoiler]
Otyugh Hole (3,000)
Cloak of Charisma + 6 (36,000)
Intelligent +1 Mithral Buckler of battle (Buckler with the belt of battle effect as per magic item compendium and DMs guide, this allows us a legendary belt of battle effect that can activate itself) (up to 41,000 and some experience)
4 Greater Metamagic rods, Energy Admixture (practically one of each, this gives us 12 admixtures) (680,000)
[/spoiler]
Known Spells (only the important ones mentioned)
[spoiler]Shape Change, Magic Missile, Arcane Fusion, Greater Arcane Fusion, Dispel Magic, Greater Dispel Magic, Greater Celerity[/spoiler]
Damage for each Missile:
[spoiler]Maximized Empowered Energy Admixtured Missile:
1 energy admixtures adds +100%, empower adds +50%, lets not assume they double eachother (that would be sick)
(5 + 5)*2.5 = 25
Maximized Empowered Missile:
(5+5)*1.5 = 15
Empowered Energy Admixtured Missile:
(3.5+5)*2.5=21.25
Empowered Missile:
(3.5+5)*1.5=12.75
[/spoiler]
Magic Missile spells per arcane fusion:
[spoiler]
Each Twinned Repeating Arcane fusion: (remember M is maximised and empowered magic missiles, E is empowered but not maxmised)
2 Empowered, twinned repeating (+possibly energy admixtured by rods) Magic Missiles
2 Maximized, Empowered, Twinned, Repeating (+possibly energy admixtured by rods) Magic Missiles
So 4 of each from that (since twinned missiles).
(+ double that in the turn after (since both the arcane fusion repeats, and the missiles does as well))
Twinned, Repeating Greater arcane fusion:
2 Twinned Repeating Arcane Fusion, 2 Twinned Repeating Empowered Maximized Magic Missile
Twinned Arcane fusion gives 4E + 4M. (Double this as it is two castings (due to greater arcane fusion being twinned) gives us 8E + 8M.
Also in a twinned repeating greater arcane fusion, we get 2 twinned repeating Empowered Maximized Magic Missile spells, which is 4M.
This gives us 12M + 8E For each Greater arcane fusion.
(however in the turn after, we would have 12M+8E (greater arcane fusion repeats) + 8M + 8E (Arcane Fusion repeats) + 12M + 8E (Missiles repeating) = 32M+24E)[/spoiler]
Missiles per casting: 6
Damage for each Magic Missile spell:
[spoiler]Maximized Empowered Energy Admixtured Magic Missile Spell
MA=6*(5 + 5)*2.5 =150
Maximized Empowered Magic Missile Spell
M=6*(5+5)*1.5 = 90
Empowered Energy Admixtured Magic Missile Spell:
EA=6*(3.5+5)*2.5=127.5
Empowered Magic Missile Spell:
E=6*(3.5+5)*1.5=76.5[/spoiler]
Casting order:
[spoiler]1st Round:
Shapechange into a Choker, This gives you dual actions.
2nd Round:
[spoiler] (REMEMBER DUAL ACTIONS DUE TO CHOKER:
Cast Energy Admixtured Quickened Twinned Repeating Greater Arcane Fusion (Swift Action Used)
Cast Energy Admixtured Twinned Repeating Greater Arcane Fusion (Standard Action used)
Cast Energy Admixtured Quickened Twinned Repeating Greater Arcane Fusion (Swift Action Used)
Cast Energy Admixtured Twinned Repeating Greater Arcane Fusion (Standard Action used)
Belt of Battle Activates, +1 Full Round Action:
Cast Energy Admixtured Quickened Twinned Repeating Greater Arcane Fusion (Swift Action Used)
Cast Energy Admixtured Twinned Repeating Greater Arcane Fusion (Standard Action used)[/spoiler]
6 Castings of Energy Admixtured greater arcane fusion nets us 6*(12MA + 8EA)=72MA+48EA
So in total round one 72MA+48EA
3rd Round
[spoiler]REMEMBER DUAL ACTIONS DUE TO CHOKER:
Cast Energy Admixtured Quickened Twinned Repeating Greater Arcane Fusion (Swift Action Used)
Cast Energy Admixtured Twinned Repeating Greater Arcane Fusion (Standard Action used)
Cast Energy Admixtured Quickened Twinned Repeating Greater Arcane Fusion (Swift Action Used)
Cast Energy Admixtured Twinned Repeating Greater Arcane Fusion (Standard Action used)
After round is over, but before anyone else gets to act, activate greater celerity:
Cast Energy Admixtured Quickened Twinned Repeating Greater Arcane Fusion (Swift Action Used)
Cast Energy Admixtured Twinned Repeating Greater Arcane Fusion (Standard Action used)[/spoiler]
6 Castings of Energy Admixtured greater arcane fusion nets us 6*(12MA + 8EA)=72MA+48EA
Then there's the carryover from last round: Earlier we calculated that each repeating greater arcane fusion spell cast in the turn before nets 32M+24E, so 6*(32MA+24EA)=192MA+144EA
In total we get 264MA+192EA On round 3.
Calculating round 3 Damage:
264*150+192*127.5=64080
[/spoiler]
So 64080 damage in one round. Damage for highest damaging missile is 25, Pretty decent damage output.
This build uses 13 Level 8 spells and one level 9 spell. But that is attainable as Sorc level 19.
Ps. How long does it take to change a belt? With Timestop one could technically throw off three greater arcane fusion with the belt, use timestop, change belt to a new belt of battle, wait for time to start, use three greater arcane fusions, then timestop, until you run out of level 9 spells (or belts, whichever comes first).
After this he will have burned 8 Level 8 spells and one level 9 spell. However, all in all he will have 12 Level 8 and 9 spells so it can be done once per day.
PPS. In theory we could have more if we did the action like: Round 2, 4 Gr. Arcane Fusion. Round 3, 4 Gr. Arcane fusion + Belt of battle for 6 Gr. Arcane fusion. Round 4, 4+2 Gr. Arcane fusion (with Celerity), since it would give us some residual spells from the first round as well (since a greater arcane fusion will have repeated the round before, and all those spells will have repeating), but I'm not in the mood to re-do all the calculations for this.