Here is the full list of all 39 LEGO The Hobbit achievements. It takes around 35-40 hours to unlock all of the achievements on Windows.
Achieve 100% completion
Complete all the quests in Middle-earth. (Single Player Only)
Collect all characters. (Single Player Only)
Forge all the Perfect Mithril Treasure Items. (Single Player Only)
Forge all the Mithril Treasure Items. (Single Player Only)
Collect 10,000,000,000 studs. (Single Player Only)
Play a level in co-op
Collect all Mithril Bricks. (Single Player Only)
Collect all Schematics. (Single Player Only)
Equip yourself with Dazzle Wig, Mithril Rhythm Stick and Mithril Dance Boots.
Collect all Minikits. (Single Player Only)
Collect all Treasure Items in levels. (Single Player Only)
Obtain all Master Burglar titles. (Single Player Only)
Equip Thorin with the Mithril Mushroom Crown
Create a custom character.
Complete the Bonus Level
Complete the mining game perfectly 50 times
Use every Dwarf from Thorin's Company in a Buddy-Up attack
Use 50 Buddy-Up attacks
Obtain 150,000 studs from Instruction Builds bonuses. (Single Player Only)
Transform to Beorn's bear form.
Use Gandalf's startle ability to dazzle 50 enemies
Complete ''Inside Information''
Complete ''On the Doorstep''
Complete ''The Necromancer''
Complete ''Looking for Proof''
Complete ''A Warm Welcome''
Complete ''Barrels Out of Bond''
Complete ''Flies and Spiders''
Complete ''Queer Lodgings''
Complete ''Out of the Frying Pan…''
Complete ''Goblin Town''
Complete ''Over Hill and Under Hill''
Collect 99 of any loot
Complete ''The Troll Hoard''
Complete ''Roast Mutton''
Complete ''Azog the Defiler''
Complete ''An Unexpected Party''
Complete ''Greatest Kingdom in Middle-earth''