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