Here is the full list of all 13 Nidhogg 2 achievements.
Beat the singleplayer in under 20 minutes
Finish an 8 player tournament
Trap your opponent on your sword for a long time while moving it up and down
play a match with old school settings
Win the faceoff and never lose the arrow in a singleplayer match
Beat the singleplayer game
Get eaten by the nidhogg after nearly allowing your opponent to do the same
100 total kills in one match
Kill yourself with your own arrow
Dominate every level in one singleplayer game
Let the timer run out in your opponent's last screen and win in sudden death
Play a match lasting longer than 20 minutes
Win 100 ranked games through matchmaking