Everyone's pretty much covered everything. Tali, Legion, or Kasumi needs to go in the vents. Miranda or Garrus needs to be in charge of the second fire team (I always choose Garrus myself). Samara or Jack needs to be in charge of the Biotic field. Anyone who is loyal can escort the crew back, although it is advised that a less combat-centered person do so. Mordin and the like. Holding the line is checked by your combat squad members. If you leave the tougher ones behind, like Grunt, Garrus, and Zaeed, the odds of everyone living go up. To insure that no one dies, leave behind all the fighters and take tech specialists with you, like Tali, Mordin, or Kasumi. Since the final battle is pretty much all you and your teammates don't help worth jack, it doesn't matter if they're not the best. Be warned: If you insist on taking Grunt with you, your odds of having everyone survive drop. It is recommended that you take the weakest person with you as well if you take Grunt.
Also, as pointed out, everyone has to be loyal. You will loose the loyalty of some party members after you complete their mission, but you side with their opposite (Jack vs Mirranda, Tali vs Legion). Hope this helps.