For Sorcerers, finding ways to selectively apply the casting time extension for metamagic (Rapid Metamagic, Instant Metamagic ACF, etc) can allow you to do truly silly things, like twinned repeating greater arcane fusion x2, each round.
In this hypothetical scenario, we're a Sorcerer-based Incantatrix 10 with Arcane Thesis (greater arcane fusion and orb of force), Practical Metamagic (Twin Spell and Repeat Spell) and Easy Metamagic (Twin Spell) (yes, it's feat-intensive, but we have bonus metamagic feats, so we can spare a couple). Now we can cast twinned repeating greater arcane fusion as an 8th-level spell. Let's say that inside our GAF we put a twinned repeating arcane fusion (containing a twinned repeating orb of force and some random first-level spell) and a twinned repeating orb of force. Let's assume we cast two of these GAFs in the first round. Anyone care to wager just how much firepower we're about to discharge?
Round 1:
Step 1) Two twinned repeating greater arcane fusions = Four repeating greater arcane fusions, each containing one twinned repeating arcane fusion and one twinned repeating orb of force.
Step 2) Four twinned repeating arcane fusions = Eight repeating arcane fusions, each containing one twinned repeating orb of force and one 1st-level spell.
Step 3) Twelve twinned repeating orbs of force = Twenty-four repeating orbs of force.
That's 240d6 force damage in the first round of combat, plus whatever our eight random 1st-level spells were. But wait, we're not done. Because next round, we've got a whole new rotation of spells that are going off!
Round 2:
Step 1) Four greater arcane fusions, each containing one twinned repeating arcane fusion and one twinned repeating orb of force.
Step 2) Eight arcane fusions, each containing one (1) twinned repeating orb of force and one 1st-level spell.
Step 3) Twenty-four orbs of force.
There. We've resolved all of our Repeating spells from Round 1. However, we have some new spells this round, thanks to our G/AFs. So, let's resolve those now.
Step 1) Four twinned repeating arcane fusions = eight repeating arcane fusions, each containing one twinned repeating orb of force and one 1st-level spell.
Step 2) Eight twinned repeating orbs of force = Sixteen repeating orbs of force.
Ok, our tally now sits at 240d6 + 400d6 = 640d6 force damage. Let's keep going, shall we?
Round 3:
Step 1) Eight arcane fusions, each containing one twinned repeating orb of force and one 1st-level spell.
Step 2) Sixteen orbs of force
Alright, we've resolved the repeating spells from last round. Let's resolve the new spells from this round.
Step 1) Eight twinned repeating orbs of force = sixteen repeating orbs of force
We're now up to 240d6 + 400d6 + 320d6 = 960d6 force damage. On to the last round!
Round 4:
Step 1) Sixteen orbs of force.
Whew, all done. Our final count: 240d6 + 400d6 + 320d6 + 160d6 = 1120d6, or 3920 avg., force damage. Not a record damage number by any means, but for two 8th-level spell slots it isn't too shabby. We still killed just about anything outside of the Force Dragons. And the neat bit? We can spread that pain around in 112 discrete packages, so if we're up against a horde of mooks, we just took them all out. And this isn't even counting all those 1st-level spells we shoved in there. We have 32 of those, whatever they were. Lesser orbs of blah, maybe, or true strikes to help some of our orbs hit home, or maybe a whole bunch of obscuring mist, because we can always use more smoke.
The nice part is that we can do that 4-5 times a day with a decent Charisma score, a couple memento magicas, or maybe a rod of absorption, and we still have all of our other spell slots untouched.
Arcane fusion and greater arcane fusion are the reasons why Sorcerers do blasting better than Wizards, and arcane spellsurge lets you double up each round, spitting out twice as much crazy.
