Are you allowed to use the Twist the Knife advanced slayer talent at 10 (it’s 3rd party)?
And also, why short swords instead of scimitars? With keen scimitars, you could have 15-20/x2 crit range, and 15-20/x3 with Twist the Knife.
Your DM is correct, those get added to the list that you may select from when gaining Ranger combat styles at lvls 2, 6 and 10.
The following quote is from: http://www.herosheets.com/the-pathfinder-rogue-top-5-weapon-enchantments/
“+3 : Speed : Every full round attack gets you an extra attack, just like you were hasted. You only need one weapon with Speed, but you won’t have to worry about staying hasted any more. Rogues are all about adding attacks to stack up Sneak Attacks, and this one adds an attack a the full bonus. At level 9, for example, you’re talking 5 attacks: -2/-2/-5/-5/-10. That’s a potential sneak attack of +25D6, not counting your weapon damage.
+2 Advancing Positional attacks are quite good for TWF Pathfinder Rogues. Our damage output is completely crippled if we can’t flank our enemies. Abilities like Advancing help a ton! Many times, your initial move won’t put you behind your enemy. Whack him, advance behind him, and hit him with a Sneak Attack on the next turn. Better yet, stack it with…
+2 Anchoring This combination should be a obvious. Anchoring on one weapon, Advancing on the other. Stick ’em twice, move behind them and lock them into place. Sneak Attacks for everyone!
+1 Keen Quite a nice choice for a low level weapon. Keen doubles your threat range – plenty of rogues will have feats that improve Attacks of Opportunity, and this goes well with them. More range is better tactical advantage, and that’s what the TWF rogue is all about
+N Any Damage Increasing Bonus – There are too many of these to list. If you’re given the choice of an enchantment on a +1 weapon, any of these – Shock, Frost, Flaming, etc are good choices. More damage is always a good thing. I would always tend towards tactical advantages, but never turn down extra damage.”
And also, why short swords instead of scimitars?
Well, with Scimitars you'd suffer a -4/-4 penalty on attack rolls, whereas with Kukri, you'd still get the 18-20 crit range(or 15-20 with keen), but only suffer a -2/-2 penalty on attack rolls.
So the question is, why not Kukri?
And as to what your GM said regarding the Ranger Combat Style Feats: You can take the Ranger Combat Style Slayer Talent at 2nd, 6th and 10th level.(you can take it at 4th level as well, IF you haven't taken it at 2nd level, and choose from the 1st tier of feats.) If you take it at 6th level, you can choose from the second tier of feats as well, and yes, you do get a feat, such as Improved Two-weapon Fighting. And at 10th level, where you can pick Greater Two-weapon Fighting. Which means you can actually get Greater Two-weapon fighting one level before what you should normally be able to, which is pretty nice.
Weapon Mods:
Before talking about weapon abilities, you could also take a look at weapon mods(basically additional upgrades that can be done to weapons, aside from enhancements and weapon abilities.)
And since you are going for Two-Weapon Fighting, Dual-Balanced weapons could be nice for you. It reduces the Two-Weapon Fighting penalty by 1. So you'd go from -2/-2(with light off-hand weapon) to -1/-1. Which isn't bad. But it costs 2000GP per weapon. And they have to be in pairs. So that's 4000GP for two weapons, which isn't much at higher levels, but a considerable amount at lower levels.
Weapon Abilities:
+1 Keen Weapons are always nice, if you want to crit a lot.
+1 Flaming, Frost or Shocking weapons are also nice, since you attack alot, that's a lot of extra damage dice. P.S: this is weak against elemental resistances! So maybe have different abilties on each weapon. One shocking, one flaming or frost and so on...
Hope this helps!
If you're going to do Circling Mongoose/Canny Tumble to gain your Sneak Attacks, you're going to be heavily reliant upon making as many Full-Round Actions as possible. Plan on doing something to get Haste somehow so you can make your move actions really count (when you have to make them), and your Full Attack Actions will really count when you're able to make them. Also, consider getting Step Up & Strike, or at least Step Up. With Step Up & Strike, you can make 10 ft steps to follow their 5 ft step (which allows you to flank better if you're clever with your 10ft steps). Step Up is really nice to have too though; if the BBEG Wizard takes a 5ft step away from you, he can cast while not being threatened (so no Conc check and no AoO), and Step Up can ensure that if a nasty caster does try to do that, you can follow him and keep him threatened.
You need Weapon Focus: Kukri in order to get Twist the Knife. Circling Mongoose seems really good, but it's expensive (5 feats/talents with max Acrobatics in order to guarantee flanks) when you could do it with either Shatter Defenses/Cornugon Smash (req 13str & 3 feats/talents with max Intimidate) or Lightning Feint (4 feats/talents with max Bluff). Power Attack and Piranha Strike are interchangeable and will be a part of every build, so I'm not considering Shatter Defenses/Cornugon Smash as a 4-feat/talent buy.
And honestly, if you're going to TWF, it's the best time to go Quick Dirty Trick + Dirty Trick Master. It only eats up one of your attacks attack to make a Quick Dirty Trick and it's some of the nastiest debuffing in the game (Blind, Nauseate, etc.)
15pt buy - Human Slayer
Str 10
Dex 18(+2human)
Con 12
Int 10
Wis 10
Cha 13
Enchants:
Keen (+1)
Agile (+1)
Skills:
Stealth (Max)
Bluff (Max)
Traits:
Dangerously Curious - for UMD so you can buff yourself wands and scrolls
Child of the Moon - +1, +2 or +4 Stealth, depending on the Moon Cycle.
For TWF, get two Dual-Balanced, Keen, Agile Kukri. Get another Kukri with a +2 Anchoring enchantment on it and start combat with it in your left hand. Stick your target in the back, activate the anchoring enchantment, and draw your proper Left Kukri, and either carve him like a turkey while he can't move or if you want you can go attack something else.
1lv Weapon Finesse
1lv: Skill Focus: Stealth (Focused Study in place of Human Bonus Feat)
2lv: (slayer talent) RCS: TWF
3lv: Dirty Fighting
4lv (slayer talent): Uncanny Feint (Improved Feint)
5lv: Improved Dirty Trick
6lv: Rogue Weapon Training: (Weapon Focus: Kukri)
6lv (extra slayer talent): ranger style=>improved TWF
7lv: Quick Dirty Trick
8lv (slayer talent): combat trick=>Piranha Strike, Human Bonus Feat (Skill Focus: Bluff)
9lv: Greater Dirty trick
10lv (Slayer Advanced Talent): Twist the Knife
11lv Greater Feint
12lv (Slayer Advanced Talent): Lightning Feint
12lv (extra slayer talent): Feat: Dirty Trick Master
13lvl: Critical Focus
14lvl: Opportunist
15lvl: Censoring Critical (can't yell for help or cast spells for 1d4+1 rounds)
16lvl: Feat: Combat Reflexes Human Bonus Feat: Skill Focus: Perception
At lvl 12 with no items or buffs:
Hit: 12BAB, 5dex, +1 WeapFocus, -4Piranha, -1 TWF, +3 Enchantment = +16 to hit (+19 vs Studied Target) with 15-20/x3
Dirty Trick CMB: 12BAB, 5Dex, +2ImpDT, +2GreDT = +21CMB (+25CMB while Flanking)
Dmg: 1d4 +5Agile +8Piranha, +3 Enchantment = 1d4+16 (+19 vs Studied Target) +4d6 (Sneak Attack vs. Flat-footed) (51-60dmg on crit, 60-69dmg on crit vs Studied Target)
AC: 10 + 5dex + 3 Studded Leather (+3 enchantment) + 1 Buckler (+3 Enchantment) = 25 AC
Stealth: 5dex + 12SkillRanks + 3ClassSkill + 6SkillFocus + 1to4 ChildOfTheMoon Trait = 27-30 Stealth (+30-33 vs Studied Target)
Bluff: 1Cha + 12SkillRanks + 3ClassSkill + 6SkillFocus = 22 Bluff (+25 vs Studied Target)
I'm considering that your Dirty Trick CMB is applying Weapon Finesse because you'll either be using your Kukri or some type of unarmed strike, like spitting in their eyes or kicking dirt to blind, or kicking/kneeing them in the kidneys/unmentionables to sicken/nauseate (Weapon Finesse applies to unarmed strikes because they're considered light weapons)
With a wand of Invisibility, you'd have a 67-70 Stealth while staying still, and a 47-50 Stealth while moving.
.
With this build, you'll be Feinting as a swift action, critting 30% of the time at 15-20/x3 with 5 attacks per round, and one dirty trick maneuver per round. Whenever you successfully feint, all of those attacks will do sneak attack damage (at lvl 12, you'll be using a +25 to Bluff vs 10+BAB+WisMod -- so the highest you might see is 10+12BAB+6Wis= 28). You're a full BAB class with Studied Target bonus to hit/dmg, and you'll be attacking Flat-Footed AC, you're going to hit hard and hit often with a +21QDT/+19/+14/+14/+9 to hit, and you can Dirty Trick Debuff your enemies right into oblivion. Basically, Blind them in the first round, Sicken them next round if they don't take a Standard to remove the Blindness. If they don't take their Standard Action to remove the Sickened condition, next round you Nauseate them, and now they can only make Move Actions (no attacks or casting) and they're Blind.
Assuming you're facing a lvl 12 monster with an AC of 27 and a flat-footed AC of 22, and assuming that you have a 100% chance to Bluff (because you'll be hard pressed to find a Full BAB mob with a Wisdom mod of 5+.... even with a Wis mod of 4, you can roll a 1 on your bluff and still beat them) your DPR with no buffs or items would be:
First Attack:
0.9*(21.5)+0.3*0.9(43)+(14)
19.35+11.61+14 = 44.96
Second Attack:
0.9*(21.5)+0.3*0.9(43)+(14)
19.35+11.61+14 = 44.96
Third Attack:
0.65*(21.5)+0.3*0.65(43)+(14)
13.975+8.385+14 = 36.36
Fourth Attack:
0.65*(21.5)+0.3*0.65(43)+(14)
13.975+8.385+14 = 36.36
Fifth Attack:
0.4*(21.5)+0.3*0.4(43)+(14)
8.6+5.16+14 = 27.76
=190.4 DPR, or 145.44 DPR + Quick Dirty Trick
That previous example was with a 15-pt buy on stats, no items or buffs (other than the weapon enchantments). So lets try a 25-pt buy, with a Belt of Incredible Dex +6, Boots of Speed (haste), Wand of Invisibility, and a Potion of Heroism.
Str10, Dex29 (+2 human, +3lvl, +6belt), Con12, Int12, Wis12, Cha12
At lvl 12 with Belt, Boots (with haste activated), Wand and Potion:
Hit: 12BAB, 9dex, +1 WeapFocus, -4Piranha, -1 TWF, +3 Enchantment, +1haste, +2heroism = +23 to hit (+26 vs Studied Target) with 15-20/x3
Dirty Trick CMB: 12BAB, 9Dex, +2ImpDT, +2GreDT = +25CMB (+29CMB while Flanking)
Dmg: 1d4 +9Agile +8Piranha, +3 Enchantment = 1d4+20 (+23 vs Studied Target) +4d6 (Sneak Attack vs. Flat-footed) (63-72dmg on crit, 72-81dmg on crit vs Studied Target)
AC: 10 + 9dex +1haste +3 Studded Leather (+3 enchantment) + 1 Buckler (+3 Enchantment) = 30 AC
Stealth: 9dex + 12SkillRanks + 3ClassSkill + 6SkillFocus + 1to4 ChildOfTheMoon Trait +2heroism +40invis = 73-76 Stealth (+76-79 vs Studied Target) while standing still, or 53-56 (56-59 vs Studied target) while moving
Bluff: 1Cha + 12SkillRanks + 3ClassSkill + 6SkillFocus = 22 Bluff (+25 vs Studied Target)
.
Hit: +26/+26/+26/+21/+21/+16 with 15-20/x3
Dirty Trick CMB: +25CMB (+29CMB while Flanking)
Dmg: 1d4+23 +4d6 Sneak Attack
AC: 30
Stealth: +73-76 Stealth (+76-79 vs Studied Target) while standing still, or +53-56 (+56-59 vs Studied target) while moving
Bluff: +25
.
Assuming you're facing a lvl 12 monster with an AC of 27 and a flat-footed AC of 22, and still assuming that you have a 100% chance to Bluff.
Haste Attack:
0.95*(25.5)+0.3*0.95(51)+(14)
24.225+14.535+14 = 52.76
First Attack:
0.95*(25.5)+0.3*0.95(51)+(14)
24.225+14.535+14 = 52.76
Second Attack:
0.95*(25.5)+0.3*0.95(51)+(14)
24.225+14.535+14 = 52.76
Third Attack:
0.95*(25.5)+0.3*0.95(51)+(14)
24.225+14.535+14 = 52.76
Fourth Attack:
0.95*(25.5)+0.3*0.95(51)+(14)
24.225+14.535+14 = 52.76
Fifth Attack:
0.75*(25.5)+0.3*0.75(51)+(14)
19.125+11.475+14 = 44.6
=308.4 DPR
or 255.64 DPR + Quick Dirty Trick