If you're having a hard time completing the No One Left Behind achievement, these are just a few tips to ensure everyone gets back alive.
1. For the sake of the achievement, you need to recruit all available companions. These DON'T include the DLC companions. To ensure success, get every companion loyal, which means completing each loyalty mission for every companion. Some questions people have had with loyalty, have been about Zaeed, and the Miranda/Jack situation. Speaking from a personal experience, in my first play through I got this achievement with NOT having Zaeed loyal, and also NOT having Jack loyal. If you have companions who aren't loyal, just don't put them in any main roles, which will come up later. (See #3)
2. Acquire all ship upgrades, which include: -Thanix Cannon, 15,000 Platinum, from Garrus -Heavy Ship Armor, 15,000 Palladium, from Jacob -Multicore Shielding, 15,000 Palladium, from Tali -Advanced Mineral Scanner, 15,000 Iridium, from Miranda -Modular Probe Bay, 15,000 Iridium, from Thane -Extended Fuel Cells, 3,000 Element Zero, from Samara
To be completely honest, the only upgrades that are ABSOLUTELY NECESSARY are the first three listed. In my play through, I acquired all six just to be safe. You may do as you wish.
3. Throughout the Suicide Mission, you will need to choose companions for special roles that will help you along. These are all the roles, and the characters you should pick to ensure survival.
-First, a "Specialist Technician" is required. Ideal: Tali/Legion -Next a "Fire-team Leader" is required for the second team that will be splitting off with yours. Ideal: Garrus, Jacob, or Miranda -After some time of fighting, you'll need a "Biotic Specialist". Ideal: Samara/Morinth or Jack. -Another Fire-team will be needed. Ideal: Garrus, Jacob, or Miranda. -Last, you'll need an escort to guide the Normandy crew back to the ship. Ideal: Mordin! He will have a very high chance of death, and this role will ensure he lives.
And remember, when picking the roles of your companions, MAKE SURE WHOEVER YOU PICK IS LOYAL! Loyality is the key to this mission, and this achievement!