Never underestimate the benefits of strategic placement, initiative, and retreat. I've found that on the 'dungeon crawl' segments of the game where I have to get through a large structure to an objective (most of the game) the best strategy is to have someone use stealth to scout out the path while everyone else stays back. Find the enemy, lay some traps, then draw them into an ambush with your characters placed properly. A tank at a chokepoint is more successful than one standing in the middle of a room and enemies advancing on you through a hail of arrows (everyone whips out some ranged weapon and contributes) then the less I have to deal with in the resulting scrum.

Make sure tactics are set so people are targeting the people getting tanked (either enemy target = tank or tank target = enemy). Maintain spacing and if something rushes past the tank don't hesitate to stun/CC the renegade and refocus the tank on it to get it back under control. Pulling back if your front line (the melee fighters) is breached is also a good idea since its very easy to get flanked and overwhelmed if multiple enemies attack at once. Depending on your talents remember to engage enemies in a systematic order with your tank's positioning as key. With skills you can prevent getting 'flanked' on the side your warrior is holding his shield and later in whichever 180 degree direction he is facing. Turning the wrong way and getting flanked by engaging enemies on your shield side early on can get your tank killed since he'll take extra damage and even backstab from rogues.

Of course random encounters and post conversation set pieces defy your abilities to plan ahead since they just group you up and place you in melee with whatever is trying to kill you. I've had the annoying problem of always having my squishies (mage and healer) in front of my warrior and rogue whenever a major boss fight begins. -_-