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.
Win the General's League
Eliminate 50 players
Get promoted to the General's League
Get promoted to the Colonel's League
Get promoted to the Major's League
Win 3 games in a row
Conquer 100 territories with one defender in
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
Win a game in which you were last in the player order
Conquer 7 or more territories in one turn
Attack 100 territories with at least twice as many troops as the defender has
Deploy 20 troops in one turn
Control a continent by the end of your 1st turn
Reach turn 5 without losing a battle as the attacker
Complete a game with at least 2 local human players
Attack with 10 or more troops
Spend 5 stars in a single purchase of troops
Conquer a territory taking no casualties