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