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 ''Greatest Kingdom in Middle-earth''
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)
Achieve 100% completion
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.
Play a level in co-op
Collect all characters. (Single Player Only)
Collect all Minikits. (Single Player Only)
Complete the mining game perfectly 50 times
Complete ''Goblin Town''
Complete ''Over Hill and Under Hill''
Equip Thorin with the Mithril Mushroom Crown
Transform to Beorn's bear form.
Complete ''The Troll Hoard''
Complete ''Roast Mutton''
Complete ''Azog the Defiler''
Create a custom character.
Complete ''An Unexpected Party''
Complete the Bonus Level
Complete ''Inside Information''
Complete ''On the Doorstep''
Complete ''The Necromancer''
Complete ''Looking for Proof''
Complete ''A Warm Welcome''
Complete ''Barrels Out of Bond''
Obtain 150,000 studs from Instruction Builds bonuses. (Single Player Only)
Complete ''Flies and Spiders''
Complete ''Queer Lodgings''
Complete ''Out of the Frying Pan…''
Collect 99 of any loot