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)
Use the Ent Draught on Pippin.
Turn an enemy into a skeleton and make them dance.
Travel to Isengard as every playable Hobbit.
Unlock all the Map Stones in Middle-earth. (Single Player Only)
Get 100%. (Single Player Only)
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)
Complete the Bonus Level.
Travel to Trollshaws as Bilbo.
Fish perfectly 20 times by pressing the icon as it flashes.
Collect all the Red Bricks. (Single Player Only)
Achieve True Adventurer in every level. (Single Player Only)
Craft the Fire and Ice Bows. (Single Player Only)
Throw Gimli 30 times.
Dress a character completely in treasure items.
Walk into Mordor.
During 'Prologue', jump into the fires of Mount Doom as Isildur.
Equip Aragorn with the Mushroom Crown.
Defeat Gollum as Gollum.
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 a level in co-op.
Complete 'Osgiliath'.
Complete 'The Dead Marshes'.
Complete 'Helm's Deep'.
Defeat 42 Uruk-hai as Legolas.
Finish one of Gimli's opponents as Legolas in 'Helm's Deep'.
Complete 'Warg Attack'.
Complete 'Track Hobbits'.
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 Mines of Moria' in under 15 minutes.
Complete 'The Pass of Caradhras'.
Form The Fellowship of the Ring.
Complete 'Weathertop'.
Complete 'The Black Rider'.
Complete 'Prologue'.