Here is the full list of all 20 Underdog Arena achievements.
You slapped two in a row! Yeah!
Three in a row. Quite impressive.
FIVE in a row! Were they AFK?
Completed all single player missions
You are formally an Underdog Arena alumnus or alumna. Put it on your résumé at once!
Use your shield to deflect a hit back on your opponent.
Slap another player while submerged in water.
Own another player when playing as Mip.
Stay in the air for three seconds without landing.
Don't hurt anyone for one minute (without kicking the bucket).
Get the lowest score in a game while playing as the fourth player.
End a game in a draw.
Say GG EZ after winning a game.
Get a negative score.
Let all characters finish a game, in one session.