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