Here is the full list of all 48 MX vs. ATV Reflex achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Place 1st in an Online Playlist match with 11 human opponents
Win all Face-Off Challenges in the MotoCareer
Place 3rd or higher in the National Series 3 in the MotoCareer
Place 3rd or higher in the Supercross Series 3 in the MotoCareer
Place 1st in 25 Online Playlist matches
Complete 25 Online Playlist matches
Win a Tag Mini-Game in an Online Playlist match
Finish ahead of any opponent with an Experience Level higher than yours in an Online Playlist match
Win a Snake Mini-Game in an Online Playlist match
Complete an Online Playlist match
Place 1st in all events in the MotoCareer
Earn a Gold medal in all MotoCareer Free Ride Challenges
Win all Series in the MotoCareer
Avoid 100 Wrecks
Place 3rd or higher in the Supercross Series 2 in the MotoCareer
Place 3rd or higher in the National Series 2 in the MotoCareer
Place 3rd or higher in the Omnicross Series 3 in the MotoCareer
Hold a Stoppie trick for 75 feet or longer
Hold a Wheelie trick for 150 feet or longer
Earn a judge's score of 10.0 in a Freestyle Event
Win a Freestyle Event in the MotoCareer without repeating a trick
Place 3rd or higher in the Waypoint Series 3 in the MotoCareer
Avoid a Wreck in a Race
Place 3rd or higher in the Champion Sport Track Series 3 in the MotoCareer
Place 1st in any Race on All-Time difficulty
Lap an Opponent in a Race
Earn a Gold medal in any MotoCareer Free Ride Challenge
Land a jump distance of 300 feet or greater
Place 3rd or higher in the Omnicross Series 2 in the MotoCareer
Place 3rd or higher in the Supercross Series 1 in the MotoCareer
Place 3rd or higher in the Champion Sport Track Series 2 in the MotoCareer
Complete a Race in a Machine with no body panels remaining on your vehicle
Place 3rd or higher in the Champion Sport Track Series 1 in the MotoCareer
Win 10 Holeshots
Complete a Race in a Machine with all body panels still attached to your vehicle
Place 3rd or higher in the Waypoint Series 2 in the MotoCareer
Place 3rd or higher in the Freestyle Series 3 in the MotoCareer
Perform three consecutive, unique airborne Tricks in one jump
Place 3rd or higher in the Freestyle Series 2 in the MotoCareer
Place 3rd or higher in the Freestyle Series 1 in the MotoCareer
Place 3rd or higher in the Omnicross Series 1 in the MotoCareer
Place 3rd or higher in the National Series 1 in the MotoCareer
Place 3rd or higher in the Waypoint Series 1 in the MotoCareer
Adjust a Tuning Slider and save the new setting
Complete MotoSkills 4, 5 and 6
Complete MotoSkills 1, 2 and 3
Pull off three unique tricks in a Race Event
Perform any airborne Trick and land successfully