Why teamwork wins
Pathfinder is not a game of lone heroes, and it's built that way on purpose. With only three actions a turn, no single character can set up a problem and solve it alone — but a party can. One of you knocks an enemy off balance; the next capitalizes before it recovers. That hand-off, repeated, is how fights are won.
The best groups aren't the ones with the biggest numbers. They're the ones whose turns build on each other.
Flanking
The simplest teamwork in the game: stand on opposite sides of an enemy in melee, and you both catch it — that −2 to its AC from Chapter 8 — for as long as you hold the pincer. No action, no roll, just positioning.
It sounds small. It isn't. Off-guard makes every attack from both of you land more often, and it turns on a whole category of rogue and ranger tricks that only work against a target that can't defend itself. Half of good melee movement is really about setting up a flank.
Aid
lets you lend your effort to an ally's action. On your turn you ready to help (spending an action to prepare it), and when your ally makes the check, you use your reaction to roll a check of your own against a standard DC 15. Succeed and they get a +1 circumstance bonus; critically succeed and it's +2 — rising to +3 or +4 if you're a master or legendary at what you're helping with.
It's the most flexible teamwork move there is — you can Aid an attack, a skill check, almost anything — and it turns a character's spare action and reaction into someone else's better odds. A party that remembers to Aid is quietly rolling higher all day.
Combining actions
Many of the game's best turns are two characters' actions clicking together. One character or a foe; now it's prone or held, and off-guard, for everyone else. One character to make it frightened; now every save and attack it makes is worse. A spellcaster slows it; now it gets fewer actions to answer with.
These conditions stack up across a party far faster than any one character could manage, and they compound — a frightened, off-guard, slowed enemy is barely a threat. Learning which of your actions set the table for your friends is the difference between four people fighting near each other and an actual team.
Stacking the odds
None of this requires a special build. Flanking, Aid, and a well-timed Trip or Demoralize are available to almost anyone, from level one. They're the habits that make an ordinary party punch above its weight.