Actually, you need to play through 4 times to get al of the achievements.
Playthrough #1 unlocks 2 ally achievements, hardcore mode, power gamer (lvl 50), standard stuff, codex, and 1 given set of use-specific commands (techs, biotics, weapons).
Playthrough #2 is a new-char playthrough (not playthrough #1 character) on hardcore mode. Unlocks 2 ally achievements, insane mode, 1 given set of use-specific commands (tech/biotic/weapons), and the hardcore achievement.
Playthrough #3 uses playthrough #1 save data for insane mode (since this would be the easiest). Unlocks the last 2 ally achievements, extreme power gamer, and beats the game on insane mode.
Playthrough #4 is a new char, on any mode, that finishes the tech/biotic/weapon sets that were not finished.
Therefore, you must play through roughly 3.5 times to get all the achievements. The issue is that to get the Extreme Power Gamer achievement, you must use an old char save, and use that through the game on one mode. Because of this, it prevents you from using a new character to do more of the "use a given power X amount of times".
Back from the dead, I'm afraid.
