Here is the full list of all 20 Risk achievements worth 1,000 gamerscore. It takes around 15-20 hours to unlock all of the achievements on Xbox One.
Conquer 100 territories with one defender in
Win 3 games in a row
Win a game in which you were last in the player order
Reach turn 5 without losing a battle as the attacker
Win a game in 10 turns or less
Win a game having lost no more than 3 territories.
Reach turn 4 without losing a territory
Attack 100 territories with at least twice as many troops as the defender has
Control a continent by the end of your 1st turn
Complete a game with at least 2 local human players
Conquer 7 or more territories in one turn
Attack with 10 or more troops
Deploy 20 troops in one turn
Spend 5 stars in a single purchase of troops
Conquer a territory taking no casualties
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