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