Here is the full list of all 20 Risk achievements worth 1,000 gamerscore. It takes around 8-10 hours to unlock all of the achievements on Xbox 360.
Win a game in which you were last in the player order.
Win a game in 10 turns or less.
Win a game having lost only 3 or fewer territories.
Complete a game with at least 2 local human players.
Deploy 20 troops in one turn.
Spend 5 stars in a single purchase of troops.
Conquer 7 or more territories in one turn.
Reach turn 5 without losing a battle as the attacker.
Reach turn 4 without losing a territory.
Control a continent by the end of your 1st turn.
Conquer a territory taking no casualties.
Attack with 10 or more troops.
Eliminate 50 players.
Get promoted to the Major's League.
Get promoted to the Colonel's League.
Get promoted to the General's League.
Win the General's League.
Win 3 games in a row.
Attack 100 territories using at least twice as many troops as the defender.
Conquer 100 territories which have only one defender.