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