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)
Travel to Isengard as every playable Hobbit.
Turn an enemy into a skeleton and make them dance.
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 Trollshaws as Bilbo.
Complete the Bonus Level.
Use the Ent Draught on Pippin.
Equip Aragorn with the Mushroom Crown.
Dress a character completely in treasure items.
Collect more than 10,000,000,000 studs. (Single Player Only)
Achieve True Adventurer in every level. (Single Player Only)
Defeat 42 Uruk-hai as Legolas.
Craft the Fire and Ice Bows. (Single Player Only)
Walk into Mordor.
Unlock all the Map Stones in Middle-earth. (Single Player Only)
Complete 'The Mines of Moria' in under 15 minutes.
Defeat Gollum as Gollum.
Complete 2 cooking puzzles.
Fish perfectly 20 times by pressing the icon as it flashes.
Throw Gimli 30 times.
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)
Complete 'Mount Doom'.
Complete a level in co-op.
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 'Osgiliath'.
Complete 'Helm's Deep'.
Finish one of Gimli's opponents as Legolas in 'Helm's Deep'.
Complete 'Warg Attack'.
Complete 'Track Hobbits'.
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'.