Here is the full list of all 13 Tiny Troopers Joint Ops achievements worth 1,000 gamerscore.
Your squad is fully upgraded
Zombie - Killed 100 zombie chickens
Zombie - Reached wave 25
Lost the first mission in either Campaign
Zombie - Reached wave 10
Zombie - Got a full squad
Completed the Soldier Campaign
Both campaigns completed
Completed the Special Ops Campaign
Revived fallen squad members 10 times
Earned 1,000,000 Command Points
Killed 3 enemies in an airstrike
Completed the Tutorial