First tip is RUN. There's nothing wrong with running away if out numbered. If you see a five man 'brick' sweeping around the map, then run away to find your fellow team members to form your own. Don't attack unless you have equal or greater numbers. Its a basic tip, and works wonders.
Next 'static' five man brick (or groundball), usually this happens in an open area of the map. And if the Brick contains multiple Engineers expect a 'bunker to be set up (and possibly minefields). Simple thing here is 'skirmish'. Use cover (like a wall) and just keep poping out to fire off the special. Work in pairs with one firing an expose, followed by one immediately after with an 'exploit'. You can sometimes get lucky and vaporise one player. Fifth player (hopefully a Doc or Engie) keeps overwatch on the team and fire off a heal if someone gets held. Don't just charge into a groundball, you'll probably lose.
Winning a firefight. If you die, before you hit the respawn button. Do a quick check to see how your team is doing. If its winning, wait for a quiet moment before hitting 'call for help'. This helps keep your team together, calling for help in the middle of a firefight is unlikely to get noticed.
Crates aren't always cover (unless you make a really tiny character). Even if you try to crouch behind one, if the enemy can see a hair on your head...then they get to fire. Use walls, and full stack of crates for cover. One other thing to look for is sometimes you can find a small gap to fire through. This limits your fire arc, but also limits the number of players than can shoot at you (be on the lookout for that one).
Expose weapon, Phasor split beam rifle OR the arcwave is pretty good. The Arcwave isn't as popular (and has a longer CD on the special). But an Arc expose with a KB is always handy for if the expose doesn't work.
Expoit weapon, there is only one...Disruptor Sniper rifle. Its got an unmarked tool tip that basically has a stacking damage buff evey time you hit. As you want to max the damage, hit with a normal shot followed by a special on an exposed target.
Kits - Carry one of everything. As the fight progresses, you can adjust your kit powers to fill in gaps on the team or exploit weaknesses in the enemy team. Many players just carry one, but sometimes that can be a mistake. For example a Tac Officer carrying a Plasma Grenade, can take out those nasty turrets while hiding behind a corner while it burns.
Thats my take (and many will disagree), but the main tip still stands....When the odds are against you...RUNNNNN!!!!!!!!!!