So, the Teamwork feat Pack Flanking reads as such:
Pack Flanking (Teamwork)
You and your companion creature are adept at fighting together against foes.
Prerequisites: Int 13, Combat Expertise, ability to acquire an animal companion.
Benefit: When you and your companion creature have this feat, your companion creature is adjacent to you or sharing your square, and you both threaten the same opponent, you are both considered to be flanking that opponent, regardless of your actual positioning.
Normal: You must be positioned opposite an ally to flank an opponent.
In the benefit it mentions that both you and your animal companion must posses this feat but the pre-reqs for the feat are an INT of 13 and Combat Expertise. I do not believe it is even possible to have the INT of my animal companion reach 13. Is this a misprint? Was there a correction somewhere to this? I can understand having my character meet the pre-reqs but it does not seem likely my AC ever will.
Honestly, I would really like to take this feat so I do not have to worry about actual flank positioning with my Mad Dog Barbarian.
It's meant to work with classes that hand out teamwork feats to their companion.
Something that grants it's teamwork feats like the Hunter and the Sacred Huntsmaster Inquisitor.
Or ones that hand it out to others, like the Cavalier's tactician ability and the dozens of archetypes that copy this.
You have observed a feat that is specifically designed for a single class and a couple of other classes that function similarly.
It is designed pretty much to only be taken by the Hunter class, and the myriad of archetypes that replicate the following Hunter class feature:
Hunter Tactics (Ex): At 3rd level, the hunter automatically grants her teamwork feats to her animal companion. The companion doesn't need to meet the prerequisites of these teamwork feats.
The feat was purposefully designed so that most classes could never access it.
Especially ones with sneak attack.
However, the gangup feat can help you to more easily achieve flanking, but it's not as easy as since you still need another creature to threaten.
Where is the Gang Up feat found?
I could not locate it in the PRD.
here has the gang up