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