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