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