Here is the full list of all 11 Who Wants to Be a Millionaire? achievements worth 1,000 gamerscore.
Reach question 5 in normal mode
Reach question 10 in normal mode
Become the champion in normal mode
Become the champion in normal mode without using a lifeline
Become the champion in easy mode
Become the champion in cooperative mode.
Answer 10 questions correctly at the very last second on the timer.
Rack up 150 seconds on the timer.
Answer more than 100 questions correctly
Answer more than 500 questions correctly
Answer more than 1000 questions correctly