Here is the full list of all 39 MX vs ATV All Out achievements.
Place 3rd or better in the Pro Freestyle Series
Successfully perform a 11x stunt combo
Place 1st in 25 Online matches
Spend over 100,000 MotoCoin
Win 10 holeshots
Place 1st in every race in a series
Equip as many factory level parts as possible on a vehicle
Upgrade a part to Factory
Spend MotoCoin
Hold a Stoppie trick for 75 feet or longer
Pull off 100 stunt combos
Lap an opponent in a race
Complete 25 Online matches
Ride for over 100 miles
Place 3rd or better in the Amateur Offroad Series
Complete any Freestyle, any Opencross, any National, and any Supercross
Wreck 50 times
Finish a race without going off track
Complete an Online match
Pull off 5 unique stunts in a race event
Land a flip or a backflipl
Land a jump with a distance of 300 feet or greater
Sucessfuly perform any stunt
Hold a Wheelie trick for 150 feet or longer
Tune a vehicle part
Place 3rd or better in the Pro UTV Opencross Series
Place 3rd or better in the Pro ATV National Series
Place 3rd or better in the Pro SX 250 West Championship
Place 3rd or better in the Pro SX 250 East Championship
Place 3rd or better in the Pro SX 450 Championship
Place 1st in any race on All Time difficulty
Place 1st in an Online match with 15 human opponents
Collect all 100 gears in Free Ride
Successfully perfrom a 7x stunt combo ending with a no hander stunt
Win a Tag Mini-Game
Survive the infection
Infect the most people
Beat your own ghost in a Time Trial event
Place 3rd or better in the All Out Tour