Here is the full list of all 19 Risk System achievements worth 1,000 gamerscore.
Achieve Rank S on Mission 1
Achieve S Rank on Mission 2
Achieve S Rank on Mission 3
Achieve S Rank on Mission 4
Achieve S Rank on Mission 5
Achieve Rank S on Mission 6
Beat the game with Ending A
Beat the game with any ending
Achieve an S Rank on all missions
Attain a combo multiplier of 128x
Attain a combo multiplier of 256
Attain a combo multiplier of 512
Hit Ashley with the barrier breaker before the fight starts
Earn a D Rank on any mission
Execute a HYPER barrier breaker
Use ace piloting skills to trick an enemy into destroying another enemy
Achieve an S Rank on the Trophaeum
Achieve an A Rank on the Trophaeum
Achieve a B Rank on the Trophaeum