Here is the full list of all 12 Who Wants to Be a Millionaire? Special Editions achievements worth 400 gamerscore.
Choose to "Walk Away" between 32,000 and 250,000 (single player game)
Choose to "Walk Away" with 500,000 (single player game)
Fail to answer the Million - dropping to 32,000 (single player game)
Get the first question wrong (single player game)
Walk away with nothing (single player game)
Use a lifeline (single player game)
Win 1,000,000 with all three lifelines remaining (single player game)
Win 1,000,000 (single player game)
Get to the Million Question - Tier 15 (single player game)
Reach 1000 (single player game)
Answer the first question correctly (single player game)
Win "Fastest Finger First" Round in under 4 seconds (single player game)