Greater Arcane fusion is an 8th level spell from Complete Mage. It lets you cast a 4th and a 7th level spell, at the same time, in the order you wish, without using up slots or casting actions. The regular version of this spell is a 5th that lets you do the same thing, but with a 4th and 1st spell.
So, Twinned repeating G.Arcane Fusion as an 8th level spell (Both metamagics are +0 for this spell, due to feats), plugging in a Twinned Repeating Arcane Fusion spell as a 7th (two +1 metamagics), and a twinned repeating empowered maximized Magic missile as a 4th level spell (Twin is 0 for MM, Empower is a +1, Maximize is a +2 due to feats). In the Twinned Repeating Arcane Fusion in your 5th level slot, you put a twinned reapeating empowered maximized magic missile as he 4th level spell, and a twinned repeating magic missile as a first level spell (Both metamagics cost 0, remember).
Round 1) Total spells cast with one standard action, using the Instant Metamagic ACF so it doesn't increase the casting time (not that it particularly matters, either way):
-One twinned Repeating Greater Arcane Fusion, containing a twinned repeating empowered maximized magic missile, and a twinned repeating arcane fusion containing a twinned repeating empowered maximized magic missile and a twinned repeating magic missile.
-Netting two twinned repeating empowered maximized magic missiles, four repeating empowered maximized magic missiles, and two repeating arcane fusions containing a twinned repeating empowered maximized magic missile and a twinned repeating magic missile each.
-Netting twelve repeating empowered maximized magic missles, and four repeating magic missiles.
That's round one. That's a total of 60 empowered maximized magic missiles, and 20 magic missiles, total. Total damage in round one is 320 +20d4 + 60d2, splittable to up to 60 different targets. That's round one.
One round two, cast the same thing again, for Twelve repeating empowered maximized magic missiles, and four repeating magic missiles, PLUS the twelve empowered maximized magic missiles, and four magic missiles that REPEATED from round one.
Total of 120 empowered maximized magic missiles, and 40 magic missiles. 640 + 40d440+ + 120d2, splittable to 120 separate targets.
60d4 + 180d2 + 960 damage in two rounds.
If you want to make sure those hit, lead in with a dispel magic via metamagic rod of quicken to drop the shield or nightshield spell the guy has. Or instead of adding Empower and Maximize (because I just picked random metamagic feats that would boost a level 1 spell to level 4), use Heighten so almost all of those magic missiles bypass a lesser globe of invulnerability thanks to being a 4th level spell. Really, any metamagic feat you want works, fell drain, whatever. If you want to be really nasty, use the Quicken rod to cast two of those GAFs in the first round, instead. Doubles your output.
There's also still four levels open at the end of the build, because you are doing this at level 16. As soon as you get Greater Arcane Fusion (Well, technically 18th, since you don't get a feat at 16th, and you have to know GAF to take Arcane fusion for it, but only if you aren't using one of the myriad ways to swap a feat. You can perform a DCFS on yourself at level 16, to swap a feat once to get it, or psychic reform, etc. after all).
