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