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.
Collect more than 10,000,000,000 studs. (Single Player Only)
Get 100%. (Single Player Only)
Dress a character completely in treasure items.
Unlock all characters. (Single Player Only)
Craft every Mithril item. (Single Player Only)
Collect all the Mithril Bricks. (Single Player Only)
Complete all the Fetch Quests in Middle-earth. (Single Player Only)
Collect all the Red Bricks. (Single Player Only)
Travel to Isengard as every playable Hobbit.
Achieve True Adventurer in every level. (Single Player Only)
Turn an enemy into a skeleton and make them dance.
Use the Ent Draught on Pippin.
Travel to Trollshaws as Bilbo.
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.
Throw Gimli 30 times.
Unlock all the Map Stones in Middle-earth. (Single Player Only)
Walk into Mordor.
During 'Prologue', jump into the fires of Mount Doom as Isildur.
Reach the top of Amon Hen without being caught once by Boromir. (Single Player Only)
Defeat Gollum as Gollum.
Equip Aragorn with the Mushroom Crown.
Complete 2 cooking puzzles.
Craft the Fire and Ice Bows. (Single Player Only)
Complete the Bonus Level.
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 'The Secret Stairs'.
Complete 'Cirith Ungol'.
Complete 'Osgiliath'.
Complete 'Helm's Deep'.
Finish one of Gimli's opponents as Legolas in 'Helm's Deep'.
Complete 'Track Hobbits'.
Complete 'Warg Attack'.
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'.
Complete a level in co-op.