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