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