In the game I'm playing in (just hit 18th level), we have pretty good teamwork.
General tactics:
*Paladin finds and locks-down the most powerful baddie.
*Rogue throws his Cloud of Daggers to hit as many enemies as possible, then flanks and dazes/stuns/initaties murderizing whatever the Paladin is targeting.
*Cleric gets to the center and throws out AoEs to take out minions/clumps of enemies and heals anyone who goes bloodied.
*Ranger chases down non-minion, non-boss enemies and takes them out one-by-one. Once all are gone, turn to help finish the boss.
*Warlock assists helps cleric with any minions, then assists the Ranger from a distance.
Some fights will cause this to change (lack of mobility, no clear "dominant" enemy, etc). Generally, the Paladin and Rogue work together and the Ranger and Warlock work together while the Cleric keeps everyone on their feet.
In the game I'm running (currently on hiatus, to resume in a few months), the party just hit 11, starting from level 3. In those 8 levels, there were 7 PC deaths, so their group composition keeps changing and, with it, the group's tactics. Only about level 7-8 or so when the party nearly TPK'd and the Swordmage was replaced by a Warden and the Pacifist Cleric became non-Pacifist did the party start to solidify tactics and greatly enhance their survivability (only 1 death in the 3 levels after the near-TPK!).
General Tactics:
*Warden gets close to the largest group of enemies possible and locks them down.
*Barbarian charges around, hitting anything that looks chargable.
*Assassin follows in the Barbarian's wake, finishing off anything that the Barbarian didn't manage to kill in one round.
*Cleric tries to stay in the middle and toss heals around.
*Sorcerer takes out minions, then focuses on enemy artillery, if possible.
There were a few exceptions, such as when they were travelling down the hollowed-out core of a massive fallen tree in the Feywild and stumbled into a mix of ettercaps and spiders. The Warden, Barbarian, and Assassin charged ahead into a nest of spiders where they were immobilized by webbing while a couple swarms and lurkers came out from behind and began hammering the Cleric and Sorcerer.
The Sorcerer and Cleric went down once or twice each before the melee front-line could fight their way back to help.
When I run my game, I tend to make fights as dynamic as possible - lurkers/"adds" that join mid fight from another direction, enemies scattered around the map in various hard-to-reach areas forcing the party to split their focus, lower-level solos mixed with elites so it's hard to tell who the "real" boss is, etc. Keeps the party on their toes and makes them revise their tactics frequently and on-the-fly.