You want to maximize sneak attack?
Everything you need is pure core.
Seriously; pure Rogue does the job (although with a dip in a PrC, you can get to 11d6 sneak attack at 20th).
Feat Tree:
1) Two-Weapon Fighting
3) Quick Draw
6) Weapon Finesse
9) Improved two-Weapon Fighting
12) Eh, Pick Something (Skill Focus (Use Magic Device) isn't horrible - and is useful to this build; see below; Improved Initiative is also a winner)
15) Greater Two-Weapon Fighting
18) Eh, Pick Something
If you drop an odd number of levels into Assasin, you get an extra d6 of sneak attack - so while a Rogue-6 would have 3d6 sneak attack, the Rogue-5/Assasin-1 would have 4d6. For the most part, though, the extra oomf isn't worth the loss of Rogue special abilities - but a Rogue doesn't have a capstone ability, so you don't need 20th.
Skills of note:
Max out Use Magic Device; it's your bread and butter skill.
How it works: You make sure every sneak attack is a touch attack.
How?
Well, you can apply Sneak Attack damage to anything with an attack roll.
Flasks of Acid (10 gp each), flasks of Alchemist's Fire (20 gp each) are both touch attacks, at range.
A Druid's Produce Flame spell is also a touch attack - either melee or ranged. Assuming you use the charges faster than the timer runs out, each attack costs you 15 gp (higher caster level groups them more, but as the number of attacks per charge is based on caster level, it's 15 gp per attack, provided you use them all up within the first 9 rounds). You want as high a caster level wand as you can muster. It's an Effect spell, so no worries about it discharging when you pull out another wand.
A Druid's Flame Blade spell is also a touch attack - melee only, and you're not proficient. Duration doesn't matter for as long as you'll have it out, so a caster level three wand will do just fine (unless your opponent has SR). 2nd level spell, though, so it starts out pricier. Plus you don't normally start out proficient with the scimitar ... but the -4 doesn't hurt too much when it's a touch attack.
A Sorcerer or Wizard's Chill Touch gives you one attack per caster level - this one isn't an effect spell, so you need to be careful with it. On the plus side, as a Touch spell, you can hold the charge forever. Again, 15 gp per attack - 1st level spell, caster level attacks per charge. Downside: You can't touch anything, as you'll waste a charge.
How's it all come together?
Well, at range, you win initiative (what, the Rogue can't win initiative? What's wrong with this picture?) and pelt the opposition with touch attacks (quickdraw flasks, usually; but Produce Flame will also work - two elements at range: Acid or Fire) - you can get a full attack in on the first round. Each one carrying those tasty, tasty sneak attack dice. And you're looking at about AC 10, in most cases, as they are flat-footed Touch attacks. If something tries to close to melee, you either kill it before it can, or use a spell that's touch range (Chill Touch, Flame Blade; Negative Energy and Fire), also from a wand.
They're all touch attacks, so even with the penalties, you're liable to hit on your last iterative attack.
How many CR 10 opponents that are subject to Sneak Attack can take the four 1d6 base + 5d6 Sneak Attack attacks that you can dish out every round at 10th? At 15th, it's six 1d6 base + 8d6 sneak attack. There's a little more or a little less depending on which exact method... but it gets sickening pretty quick.
And you've got Use Magic Device, and another 7+Intelligence Modifier skill points per level to play with, for things like, oh, Hide, Climb, Move Silently, Disable Device, Search, Open Locks, and Gather Information. So you can still do all the traditional rogue-type stuff, and play a backup caster through the use of scrolls and wands.
Do keep a backup pair of shortswords, though. And a luckblade just for the reroll.
And that's pure Core.
