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