Here is the full list of all 14 Viking Squad achievements.
Find your first piece of gear
Defeat Loki
Reach max level with one character
Find the boxers
Defeat 10000 enemies
Reach max level on all characters
Defeat the Kraken
Defeat the Yeti
Defeat the Little King
Defeat the Sentinel
Collect 10000 coins
Get defeated for the first time
Clear boss rush mode
Clear Hard Mode