Completion for all four careers is based on the same requirements as completing one career (earn 72 stars and beat the song The Thing That Should Not Be)
-- The whole career on a given instrument does not have to be defeated on one specific difficulty--if there's a song you're having trouble with, simply drop down in difficulty, or better yet, change to a different song since you only need to pass a certain number of songs to unlock The Thing That Should Not Be. While songs like Fight Fire With Fire will be unlocked after a certain point in Guitar or Bass careers, you can still go back and play MUCH easier songs like Toxicity or Stacked Actors and likely get many more stars for your trouble.
-- Vocals are a slightly different beast than in GH:WT, so it's actually somewhat enjoyable to play on higher difficulties this time around. If you're still not feeling it, place your microphone next to something that makes a constant amount of noise (a fan is best) and select Beginner as your difficulty.
-- Band career is NOT required for this achievement.