In Pathfinder, the penalties for multi-classing have been mostly been eliminated except for the favored class bonus. As a result of this, with the appropriate build, one can create an incredibly dangerous melee fighter taking advantage of the many bonus feats different classes give at early levels.

The following build is a bit of theory crafted mayhem that is incredibly effective between levels 5 and 12, so the majority of the PFS career of a player. This character is screwed towards GMs who can apply credits to characters to bypass the first several levels of play, as this character only really becomes viable at about level 4 due to a couple of 4k gold items.

Finally before we begin a warning. This build is should only be played by experienced players who know what they're doing and are willing to incur the well deserved wrath of their GM. This build has a lot of moving parts and is very easy to screw up and take excessive time if you don't know the system well.

First the stats:

Halfling

Str: 6

Dex: 20 (boost Dex every 4 levels)

Con: 12

Int: 7

Wis: 14

Cha: 14

Classes:

Level 1: Swashbuckler (Mouser)

Level 2: Fighter (unarmed fighter)

Level 3: Paladin (Iroran)

Level 4: Monk (Master of Many Styles + Kata Master)

Level 5: Monk (Master of Many Styles + Kata Master)

Level 6: Paladin (Iroran)

Level 7: Slayer

Level 8: Slayer

Level 9: Slayer

Level 10: Paladin (Iroran)

Level 11: Paladin (Iroran)

Feats:

Level 1: weapon finesse (from swashbuckler)

Level 1: Piranha strike

Level 2: Monkey Style (bonus from unarmed fighter)

Level 3: Snake Style

Level 4: Monkey Shine (bonus from Monk MoMS)

Level 5: Snake Fang (bonus from Monk MoMS)

Level 5: Combat reflexes

Level 7: Stand Still

Level 7: Ranger Combat Style: Aspect of the Beast (Claws of the Beast)

Level 9: Risky Striker

Level 11: Outflank

Next the two crucial items to this build:

Amulet of Mighty Fists (agile): This will allow you to have all of your attacks apply your dex to damage.

Ring of Seven Lovely Colors: This will allow you to transform into a tiny songbird for 10 minutes at a time, 7 times a day. Enough that you can be in songbird form for every single fight of a PFS scenario.

Ring of Eloquence: Allows you to speak with your party while in bird form.

How it all comes together:

When in bird form, you will get a +4 to dex, a -2 to strength and a +1 natural armor bonus to AC. In addition as a tiny creature you will get a +2 size modifier to hit and to AC.

Secondly, as a tiny creature, you use dex instead of strength for your CMB.

Third, and most crucially, tiny creatures can enter an enemy square freely, although this provokes attacks of opportunity. This will allow you to take advantage of the special mouser abilities as well as the feat Monkey Shine. Monkey shine is one of the key feats in this build, as it grants a +4 dodge bonus to AC when you are in an enemy square and a +4 bonus to hit while in the enemy square.

Fourth, snake style allows you to consider all unarmed strikes to be piercing weapons, making them qualify for swashbucklers finesse. In addition snake fang allows you to take an attack of opportunity every time an enemy swings against your character and misses. With an AC that will generally require almost every enemy to roll a 20 to hit you, and an almost unlimited number of AOOs, this will be a major advantage, especially since provoking every time you move in to attack will grant you an extra attack almost all of the time.

Fifth, the mouser grants the ability to flank an enemy when you are in their square and an ally is adjacent to you. This means that so long as you have another melee character in the party, you will be flanking nearly every round of combat. Additionally, the mouser can take an AOO every time an enemy leaves the mousers square, even if just using a 5 foot step. When combined with your high CMB and Stand still at level 7, you can pretty much prevent any enemy from run away from you.

Sixth, The dodging panache deed, will effectively give you the step up feat as well as a useful way of increasing your AC. The dodging panache deed allows you to take an immediate action to move 5 ft. when an enemy makes a melee attack against you and add your cha modifier as a dodge bonus to AC for a round. Since the deed does not specify which direction you need to go, you can use this as an easy way to enter an enemy square on their turn if they step back and strike at you. In addition since you should have a high cha, this should be a significant boost to your AC.

Key strengths of the build:

Ridiculous saves: Without any items, this character will have the following saves at level 12 as well as evasion:

Fort: +16

Ref: + 20

Will: +13

Ridiculous AC and touch AC:

One of the key strengths of this build is that you will be able to stack many many bonuses to your AC. First you will gain the size bonus, secondly you will get wisdom to AC from the monk. Third you will get charisma to dex AC from the Iroran Paladin (limited by number of paladin levels taken), Finally, you should have a ridiculous dex score from transforming and investing in dex boosting items. This score can be boosted and supplemented by the use of wands. I strongly encourage investing in wands of mage armor, barkskin, and shield. The level 12 character I have who is a slight variant of this build regularly has a 46 regular AC and a 36 touch AC.

Very High bonus to hit:

The combination of gaining a very high dex from changing shape, a +2 bonus to hit from tiny size, being only 1 level removed from being full BAB, and gaining a +4 to hit when in an enemy square from Monkey shine, means that this character should be able to hit most opponents on anything above a 4 most of the time from my personal experience.

Many attacks:

With this character, you will have 3 natural attacks as well as 3 regular weapon attacks at level 12. In addition, you will be able to counter-attack every attack made by the enemy who's square you are in so long as they miss. With your dex modifier, this should be enough for all of them.

High Damage per strike:

The other key benefit of this build is that in addition to having a good damage stat from dexterity, you will be able to greatly increase your damage by applying piranha strike and risky striker on all targets that are at least medium sized, which will be nearly every target you encounter.

For example at level 9, at the cost of decreasing AC by 3 and the bonus to hit by 3 you will get to add +12 to damage on each hit.

Key Weakness: anything with strength drain is a sign to run like hell. However, with your touch AC and the ability to fly, even then you should probably be fine in most cases.

I will continue to modify this and work to answer people's questions on the subject.