Here is the full list of all 48 MX vs. ATV Reflex achievements worth 1,000 gamerscore.
Place 3rd or higher in the Waypoint Series 1 in the MotoCareer
Place 3rd or higher in the National Series 1 in the MotoCareer
Place 3rd or higher in the Freestyle Series 1 in the MotoCareer
Earn a Gold medal in any MotoCareer Free Ride Challenge
Complete MotoSkills 1, 2 and 3
Complete MotoSkills 4, 5 and 6
Perform any airborne Trick and land successfully
Pull off three unique tricks in a Race Event
Avoid a Wreck in a Race
Place 3rd or higher in the Waypoint Series 2 in the MotoCareer
Place 3rd or higher in the Waypoint Series 3 in the MotoCareer
Place 3rd or higher in the National Series 2 in the MotoCareer
Place 3rd or higher in the National Series 3 in the MotoCareer
Place 3rd or higher in the Supercross Series 1 in the MotoCareer
Place 3rd or higher in the Supercross Series 2 in the MotoCareer
Place 3rd or higher in the Supercross Series 3 in the MotoCareer
Place 3rd or higher in the Freestyle Series 2 in the MotoCareer
Place 3rd or higher in the Freestyle Series 3 in the MotoCareer
Place 3rd or higher in the Omnicross Series 1 in the MotoCareer
Place 3rd or higher in the Omnicross Series 2 in the MotoCareer
Place 3rd or higher in the Omnicross Series 3 in the MotoCareer
Place 3rd or higher in the Champion Sport Track Series 1 in the MotoCareer
Place 3rd or higher in the Champion Sport Track Series 2 in the MotoCareer
Place 3rd or higher in the Champion Sport Track Series 3 in the MotoCareer
Win all Series in the MotoCareer
Win all Face-Off Challenges in the MotoCareer
Perform three consecutive, unique airborne Tricks in one jump
Earn a judge's score of 10.0 in a Freestyle Event
Lap an Opponent in a Race
Win 10 Holeshots
Adjust a Tuning Slider and save the new setting
Complete a Race in a Machine with all body panels still attached to your vehicle
Complete a Race in a Machine with no body panels remaining on your vehicle
Avoid 100 Wrecks
Land a jump distance of 300 feet or greater
Hold a Wheelie trick for 150 feet or longer
Hold a Stoppie trick for 75 feet or longer
Place 1st in all events in the MotoCareer
Earn a Gold medal in all MotoCareer Free Ride Challenges
Complete an Xbox LIVE Playlist Match
Place 1st in an Xbox LIVE Playlist Match with 11 human opponents
Complete 25 Xbox LIVE Playlist Matches
Place 1st in 25 Xbox LIVE Playlist Matches
Finish ahead of any opponent who has a higher Experience Level in a Xbox LIVE Playlist Match
Win a Tag Mini-Game in an Xbox LIVE Playlist Match
Win a Snake Mini-Game in an Xbox LIVE Playlist Match
Win a Freestyle Event in the MotoCareer without repeating a trick
Place 1st in any Race on All-Time difficulty