Here is the full list of all 48 LEGO The Lord of the Rings achievements. It takes around 30-35 hours to unlock all of the achievements on Windows.
Complete a level in co-op.
Get 100%. (Single Player Only)
Unlock all characters. (Single Player Only)
Craft every Mithril item. (Single Player Only)
Turn an enemy into a skeleton and make them dance.
Collect all the Mithril Bricks. (Single Player Only)
Complete all the Fetch Quests in Middle-earth. (Single Player Only)
Collect more than 10,000,000,000 studs. (Single Player Only)
Dress a character completely in treasure items.
Collect all the Red Bricks. (Single Player Only)
Craft the Fire and Ice Bows. (Single Player Only)
Use the Ent Draught on Pippin.
Travel to Isengard as every playable Hobbit.
Defeat Gollum as Gollum.
Travel to Trollshaws as Bilbo.
Complete the Bonus Level.
Unlock all the Map Stones in Middle-earth. (Single Player Only)
Achieve True Adventurer in every level. (Single Player Only)
Complete 'Mount Doom'.
Complete 'The Black Gate'.
Walk into Mordor.
Complete 'Cirith Ungol'.
Complete 'The Battle of Pelennor Fields'.
Defeat an Oliphaunt as Legolas.
Finish one of Gimli's opponents as Legolas in 'Helm's Deep'.
Complete 'The Mines of Moria' in under 15 minutes.
Complete 'The Paths of the Dead'.
Complete 'The Secret Stairs'.
Fish perfectly 20 times by pressing the icon as it flashes.
Complete 'Osgiliath'.
Throw Gimli 30 times.
Complete 'Helm's Deep'.
Complete 'Warg Attack'.
Complete 'Track Hobbits'.
During 'Prologue', jump into the fires of Mount Doom as Isildur.
Complete 'The Dead Marshes'.
Defeat 42 Uruk-hai as Legolas.
Complete 2 cooking puzzles.
Equip Aragorn with the Mushroom Crown.
Complete 'Taming Gollum'.
Reach the top of Amon Hen without being caught once by Boromir. (Single Player Only)
Complete 'Amon Hen'.
Complete 'The Mines of Moria'.
Complete 'The Pass of Caradhras'.
Form The Fellowship of the Ring.
Complete 'Weathertop'.
Complete 'The Black Rider'.
Complete 'Prologue'.