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.
During 'Prologue', jump into the fires of Mount Doom as Isildur.
Dress a character completely in treasure items.
Equip Aragorn with the Mushroom Crown.
Travel to Trollshaws as Bilbo.
Defeat an Oliphaunt as Legolas.
Defeat 42 Uruk-hai as Legolas.
Complete 'The Mines of Moria' in under 15 minutes.
Fish perfectly 20 times by pressing the icon as it flashes.
Travel to Isengard as every playable Hobbit.
Turn an enemy into a skeleton and make them dance.
Use the Ent Draught on Pippin.
Complete a level in co-op.
Defeat Gollum as Gollum.
Get 100%. (Single Player Only)
Collect all the Red Bricks. (Single Player Only)
Complete all the Fetch Quests in Middle-earth. (Single Player Only)
Unlock all characters. (Single Player Only)
Craft every Mithril item. (Single Player Only)
Collect all the Mithril Bricks. (Single Player Only)
Craft the Fire and Ice Bows. (Single Player Only)
Collect more than 10,000,000,000 studs. (Single Player Only)
Achieve True Adventurer in every level. (Single Player Only)
Throw Gimli 30 times.
Complete 2 cooking puzzles.
Complete the Bonus Level.
Walk into Mordor.
Unlock all the Map Stones in Middle-earth. (Single Player Only)
Complete 'Mount Doom'.
Complete 'The Black Gate'.
Complete 'The Battle of Pelennor Fields'.
Complete 'The Paths of the Dead'.
Complete 'Cirith Ungol'.
Complete 'The Secret Stairs'.
Complete 'Osgiliath'.
Complete 'Helm's Deep'.
Finish one of Gimli's opponents as Legolas in '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'.