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.
Collect more than 10,000,000,000 studs. (Single Player Only)
Complete a level in co-op.
Unlock all the Map Stones in Middle-earth. (Single Player Only)
Travel to Isengard as every playable Hobbit.
Use the Ent Draught on Pippin.
Defeat Gollum as Gollum.
Complete 'The Mines of Moria' in under 15 minutes.
Get 100%. (Single Player Only)
Travel to Trollshaws as Bilbo.
Unlock all characters. (Single Player Only)
Craft every Mithril item. (Single Player Only)
Turn an enemy into a skeleton and make them dance.
Collect all the Mithril Bricks. (Single Player Only)
Collect all the Red Bricks. (Single Player Only)
Complete all the Fetch Quests in Middle-earth. (Single Player Only)
During 'Prologue', jump into the fires of Mount Doom as Isildur.
Craft the Fire and Ice Bows. (Single Player Only)
Equip Aragorn with the Mushroom Crown.
Dress a character completely in treasure items.
Walk into Mordor.
Fish perfectly 20 times by pressing the icon as it flashes.
Achieve True Adventurer in every level. (Single Player Only)
Throw Gimli 30 times.
Defeat 42 Uruk-hai as Legolas.
Complete 2 cooking puzzles.
Complete the Bonus Level.
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 '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'.
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'.