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