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