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