So I had this idea rolling around in my head that originated with a Monk/Rogue/Scout I played in a campaign before. The idea was to boost my attacks to absurd levels of damage using unarmed strike, sneak attack, and skirmish shenanigans. And What i ended up with was a ludicrous amount of d6 damage dice and a couple d8/d10.
I'm not 100% certain on how one would go about building this to be optimized but here is the basic concept and a few, in my opinion, important building blocks.
Ok so you'll want to start off with a race & template that combines as many of the following as possible:
Extra limbs
Tail
Small size (diminutive or somehow smaller preferred - will be explained later)
Natural claw attacks
Next you'll want to take levels in any classes that provide the following and optimize for greatest results:
Sneak Attack
Sudden Strike
Skirmish
Unarmed Strike damage increase
You'll pick up the following for weapons:
Beast Claws (one per hand + 2 extra - See magic)
Rogue Blade (held in tail - see feats)
Feats:
Perfect Multiattack (& prerequisites)
Perfect Multi-weapon Fighting (& prerequisites)
Multitasking (& prerequisites)
Prehensile Tail
Craven
Telling Blow
Improved Critical (& prerequisites)
Weapon/Armor Enchantments & other Magic items:
Anything & Everything that grants you extra Sneak Attack dice and other damage dice
Magic (obtained through whatever methods you can manage):
Fell the Greatest Foe
Hunter's Eye
Girallon's Blessing
Divine Sacrifice
Now for an explanation:
So once you get everything this is how it works
Beast Claws: 1d6 spiked gauntlet that becomes 2d6+claw damage and counts as unarmed if you have a natural claw attack. Your increase in unarmed strike damage should boost your claw damage netting you an upwards of 2d6+2d10 damage from your Beast Claws. Add in any bonus damage from enchantments and you could find yourself dealing around 8d6+2d10 damage with each claw.
Rogue Blade: Grants a constant Blink effect on the wielder thus allowing you to apply your sneak attack & sudden strike on ever attack. Like the claws you'll want to add as many damage increasing enchantments as you can find to this weapon as well.
Scout & Skirmish: Get as many Skirmish dice as you can manage and be sure to move at least 10 feet each round before attacking so you can apply your Skirmish.
Sneak Attack & Sudden Strike: Like Skirmish you will want to capitalize on these abilities and due to the Rogue Blade you need only attack your target to apply the extra damage.
Craven, Telling Blow, & Improved Critical Add your character level to Sneak Attack damage, then proceed to add your Sneak Attack Damage to your critical strikes, and double your threat range.
Extra Limbs & Feats for them: With each extra limb and the appropriate set of feats you will vastly increase your damage output. Combining the Insectile Template with a race that has a Tail and gaining the effects of Girallon's Blessing will net you effectively 9 usable limbs (8 arms + 1 tail). The Prehensile Tail feat lets you grasp items and weapons with your tail and treat it as if it were an extra arm & hand. Girallon's Blessing will give you an extra set of arms and a natural claw attack per arm as well as a 2d6 Rend ability.
Small Size & Smaller: The spell Fell the Greatest Foe is an interesting spell when placed on the smallest of creatures. When attacking a creature that is larger than you you deal a bonus 1d6 damage per size category larger. That's right, the smaller you are compared to your enemy the more damage you deal.
Hunter's Eye: Perhaps one of the most important spells to have access to for this build. It only lasts for one round but in that round you gain an extra 1d6 per 3 caster levels of the spell to your Sneak Attack. Now remember, you have the feat Telling Blow, so these bonus Sneak Attack dice also increase your Critical strike damage as well. And the best part about it, the bonus dice are not capped. If you can get it high enough you shouldn't need it for more than 1 round anyway.
Odds are you won't be able to get yourself a crazy high caster level to take advantage of this with though so my suggestion would be to take a few levels of Spellthief so you can steal 2nd level spells and have a party member who is either a Ranger or an Archivist using CL shenanigans who will let you "borrow" the spell from them and cast it that way. Since Spellthiefs cast stolen spells at the CL of the spells owner you'll gain all the benefits of their over the top CL. Plus Spellthief grants 1d6 sneak attack at Lv1 so you probably already took a 1 level dip for maximizing sneak attack dice earlier a few extra levels shouldn't hurt.
Divine Sacrifice: Like Hunter's Eye you may want to cast this spell on yourself before you attack. You'll sacrifice 10HP per round/CL to deal an extra 5d6 damage on the first successful attack each round. It's not a huge bonus but every little bit helps.
Like Hunter's Eye you'll need some way to cast this spell on yourself, if you took a level or two of Spellthief then you may as well put it to use for getting this spell as well. This time however you'll need either a Paladin or Archivist to provide the spell for you. Unlike Hunter's Eye you only need the ability to steal 1st level spells for this one and since it lasts for a number of rounds = CL you don't need much else for it.
Combat: As i mentioned before I am no expert at optimizing so I'm just going to use a few base reference numbers here. If anyone can provide me with an optimized template and the numbers to go with it I'll change this section to reflect it.
Sneak Attack: 10d6 +16d6(items) + Hunter's Eye + Character Level
Sudden Strike: 10d6 +16d6(items)
Skirmish: 5d6 +4d6(items)
Unarmed Strike: 2d10 (2d8 Small)
If I recall correctly there are a couple feats that allow you to stack your Rogue, Ninja, Scout, & Monk levels to determine each of these class features and more. So maximizing all of them shouldn't be impossible...
Assuming you are of Small Size using Medium Sized weapons and attacking a Colossal enemy, full iterative attacks and critical every hit, and moved at least 10ft. I know people have quoted CLs of an upwards of 3k through all sorts of shenanigans but for the sake of example lets just say Hunter's Eye was cast at a CL of 99 for a bonus of 33d6 Sneak Attack dice, this can be updated later if someone wants to provide me some more solid numbers. And an Epic character level of say 30.
32x 8d6+2d8 claw attack
4x 2d6 Rend
4x 6d6 Rapier
1x 5d6 Divine Sacrifice
32x 59d6+2d8+30 Critical Strike
4x 57d6+2d8+30 Critical Strike
36x 51d6+30 Sneak Attack
36x 26d6 Sudden Strike
36x 9d6 Skirmish
36x 5d6 Fell the Greatest Foe
that totals up to in a single full round attack dealing over 5,685d6+136d8+2,160 damage and that's without adding in strength or any other damage modifiers you may have.
If you miraculously rolled max damage on everything then you could hit up to 27,854 damage in a single round.