Here is the full list of all 14 Just Arms achievements.
You won a multiplayer match for the DLC
You won a multiplayer match
Complete the DLC
Unlock the robot knight
Unlock the ghost knight
Unlock the frozen knight
Complete the game
Unlock the gold knight skin
Climb 2/3 of the map
Unlock the nun knight skin
Fall to the very bottom 25 times
Unlock the black knight skin
Climb 1/3 of the map
Unlock the skull knight skin