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