Here is the full list of all 12 Red Alert 3: Commander's Challenge achievements worth 200 gamerscore.
Unbelievable! You completed all 50 challenge missions under par time.
You completed all 13 main path challenge missions under par time
You cleaned up! You killed 20 Desolator Troopers.
You killed all of the giant bears in "Number One Threat to America".
Congratulations! You completed all 50 challenge missions.
You killed 20 enemy commandoes!
You completed all 13 main path challenge missions
You completed 25 challenge missions. Great job!
You killed 20 Steel Ronin
You completed 10 challenge missions. Good job!
Completed "Dead Meat"
You completed all lessons in the tutorial