Here is the full list of all 29 MX vs. ATV Supercross Encore achievements.
Place 3rd or higher in the 250 East Career
Win 10 Holeshots
Place 3rd or higher in the ATV East Career
Pull off 100 stunts
Land a jump distance of 200 feet or greater
Pull a wheelie over 150 feet
Wreck 50 times
Pull a stoppie over 25 feet
Place 3rd or higher in the ATV West Career
Place 3rd or higher in the 250 West Career
Equip an upgrade part
Pull off three unique stunts in a Race Event
Land a backflip
Lap an Opponent in a Race
Perform any airborne trick and land successfully
Finish a race without going off track
Finish 50 races online
Place 3rd or higher in the MX vs ATV East Career
Place 3rd or higher in the MX vs ATV West Career
Complete an Online Playlist match
Ride for over 100 miles in Free Ride
Place 1st in all tracks in career
Place 1st in 25 Online Playlist matches
Place 1st in any Race on All Time difficulty
Place 3rd or higher in the 450 Championship Career
Place 3rd or higher in the ATV Championship Career
Place 3rd or higher in the MX vs ATV Championship Career
Complete 25 Online Playlist matches
Place 1st in an Online Playlist match with 11 human opponents