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