Here is the full list of all 20 Ultimate Arena achievements.
Have 30 Events in your Library
Kill 50 Fighters with Landmines in a Single Game
Have a Fighter Reach -50% Sanity
Read the Credits
Click the Facebook button on the Main Menu
Click the Twitter button on the Main Menu
Have 500 Fighters in your Library
Have 1000 Fighters in your Library
Reach 1000 Updates in a Single Game
Have a Single Game last 10 Days
Have a Single Game last a Week.
Reach 80 Homicides in a Single Game
Reach 500 Updates in a Single Game
Reach 60 Homicides in a Single Game
Have 200 Fighters in your Library
Have a Fighter get off Mr. Troy's Wild Ride
Have 10 Events in your Library
Visit the Github page for Ultimate Arena
Have 100 Fighters in your Library
Kill 90 Fighters with Landmines in a Single Game