Update: Sparks Chaotix got under 3 minutes in the Sniper Rifle Challenge 2 and has beaten all the other time limits (from PS3 version of the game) as well but the achievement still didn't unlock. So my theory is wrong unfortunately :/
Just found out that the achievement probably isn't completely glitched. The challenge times in the Xbox version are wrong because it says 10 minutes for every challenge there.
These are the correct set times:
SNIPER RIFLE CHALLENGE 2
TIME TO BEAT: 3 Minutes
JET PACK CHALLENGE 2
TIME TO BEAT: 5 Minutes, 30 Seconds
SUPER SHEEP CHALLENGE 2
TIME TO BEAT: 4 Minutes 30 Seconds
SHOTGUN CHALLENGE 2
TIME TO BEAT: 2 Minutes
ACCURACY CHALLENGE 2
TIME TO BEAT: 5 Minutes, 30 Seconds
NAVIGATION CHALLENGE 2
TIME TO BEAT: 1 Minute, 20 Seconds
CRATE COLLECT CHALLENGE 2
TIME TO BEAT: 5 Minutes, 30 Seconds
DEATHMATCH CHALLENGE 11
TIME TO BEAT: 10 Minutes
The problem is that it is impossible to beat the 3 minute sniper rifle challenge #2 because the sniper rifle is affected by a glitch in the Xbox version that forces a reload after each shot (I've seen this video of the PC version ->
and there is no reloading) which wastes too much time.
1st on the Xbox Live leaderboards completed the challenge in 3:40. I haven't been able to shoot more than 30 targets in 3 minutes. If someone manages to beat this challenge in 3 minutes or less please let me know in the comments.