Here is the full list of all 14 Slash Arena: Online achievements.
Get and complete quest: "Killer"
Get and complete quest: "Unstoppable"
Get and complete quest: "No mercy"
Get and complete quest: "In the balance"
Get and complete quest: "Best of the best"
Get and complete quest: "Thief"
Get and complete quest: "Peaceful"
Get and complete quest: "Kamikadze"
Get and complete quest: "Multikill"
Get and complete quest: "Do not bully"
Get and complete quest: "Immortal"
Get and complete quest: "Clones"
Get and complete quest: "Steaks"
Get and complete quest: "Pumpkins"