Here is the full list of all 119 Torchlight II achievements. It takes around 200 hours to unlock all of the achievements on Windows.
Defeat 500 champions
Purchase 100 items from the gambler
Socket 50 gems into items
Explode 10,000 enemies
Defeat One-Eyed Willy
Die with a hardcore character
Start a hardcore character
Destroy a socketed gem
Find a legendary item
Have 50 friends
Squish 500 critters
Defeat Cacklespit
Do 10,000 damage with a single attack
Find 10 secret rooms
Have 10 friends
Take 100,000 steps
Recover 10 gems from items
Use a map
Add a friend
Trade an item with another player
Defeat the Nether King in Normal difficulty
Defeat the Nether King in Veteran difficulty
Purchase 50 items from the gambler
Defeat the Nether King in Casual difficulty
Defeat the Nether King as an Embermage
Defeat the Nether King
Start a multiplayer game
Defeat Vyrax
Defeat the Dark Alchemist
Purchase 20 items from the gambler
Defeat Killbot
Defeat Thiss
Use an enchanter 50 times
Find a secret room
Defeat Bloatfang
Have 100,000 gold
Fish 100 times
Defeat the Grizzled Alpha
Join a multiplayer game
Take 50,000 steps
Defeat the Wraith Lord
Deal 1,000 critical strikes
Obtain a tier 2 skill bonus
Defeat Boletus Rex
Teach a pet 4 spells
Learn 4 spells
Defeat the Siege Guardian
Break 1,000 breakables
Open 500 chests
Defeat Ezrek Khan
Teach a pet a spell
Defeat 100 champions
Defeat Aruk
Defeat the King In Masks
Recover a gem from an item
Defeat the Artificer
Squish 100 critters
Use 50 potions
Sell 500 items
Fish using dynamite
Defeat the Manticore's Mate
Defeat the Manticore
Use an enchanter 10 times
Defeat Grand Regent Eldrayn
Defeat Kidrik the Mauler
Defeat King Pogg
Wear a completed armor set
Defeat the Winter Widow
Explode 1,000 enemies
Defeat Chillhoof
Take 10,000 steps
Defeat 25 champions
Defeat General Grell
Open 100 chests
Defeat Mordrox
Obtain a tier 1 skill bonus
Break 100 breakables
Socket a gem into an item
Learn a spell
Send your pet to town
Defeat the Fallen Guardian
Explode 100 enemies
Fish 10 times
Open 20 chests
Defeat the Nether King with each of the 4 classes
Defeat the Nether King as a Berserker
Defeat the Nether King in Elite difficulty
Defeat the Nether King as an Engineer
Defeat the Nether King in hardcore mode (casual)
Defeat the Nether King in hardcore mode (elite)
Defeat the Nether King in hardcore mode (normal)
Defeat the Nether King in hardcore mode (veteran)
Defeat the Nether King as an Outlander
Break 10,000 breakables
Deal 10,000 critical strikes
Deal 100,000 critical strikes
Use an enchanter 500 times
Fish 1,000 times
Have 1,000,000 gold
Have 10,000,000 gold
Begin new game plus
Recover 100 gems from items
Destroy 10 socketed gems
Destroy 100 socketed gems
Find 100 secret rooms
Sell 5,000 items
Sell 50,000 items
Obtain a tier 3 skill bonus
Socket 500 gems into items
Use 10 maps
Use 100 maps
Use 500 potions
Use 5,000 potions
Assemble TrillBot
Defeat Aleera
Squish 1000 critters
Defeat Marishka
Defeat Verona
Reach level 100