Here is the full list of all 50 Viking: Battle for Asgard achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Finish off 150 Legion with fatality moves
Kill 200 Legion soldiers
Liberate the farm and the quarry in Niflberg
Finish off 100 Legion with fatality moves
Locate and kill a Legion Assassin
Kill 100 Legion soldiers
Learn a total of 4 new moves
Find and defeat a Legion Champion
Master the first new move at the Battle Arena
Kill 50 Legion soldiers
Kill 50 Legion without using a health potion
Finish off 25 Legion with fatality moves
Master a total of 8 new moves
Master a total of 12 new moves
Kill 10 Champions
Kill 5 Giants
Learn all the moves in the game
Kill 5 Legion just using Throwing axes
Lead your army to victory at Hel's Fortress without need for immortality
Kill 500 Legion soldiers
Lead your army to victory at the battle for Caldberg
Lead your army to victory at the battle for Darkwater
Lead your army to victory at the final battle for all Midgard
Lead your army to victory at the battle for Holdenfort
Lead your army to victory at the battle for Thornvik
Power-up over 500 Vikings in your armies with runic magic
Kill a Legion Champion without being hit
Use your defensive bash to knock your Legion foes to their doom!
Reach the end of the game on normal difficulty
Reach the end of the game on hard difficulty
Buy flamepots and incinerate 10 Legion soldiers with them
Complete the quest to sneak into Darkwater
Find and slaughter a Legion Giant
Liberate Niflberg completely on the normal difficulty level
Liberate Niflberg completely on the hard difficulty level
Find and charge the dragon gem Hugin and dragon amulet, and summon a dragon
Collect the 6 skulls in Darkwater
Liberate Galcliff completely on the normal difficulty level
Liberate Galcliff completely on the hard difficulty level
Complete the second dragon gem, and summon a dragon
Collect the 6 skulls in Holdenfort and the 6 skulls in Caldberg
Kill Hel's Harbinger Drakan
Liberate the farm, still, lumber mill, and the quarry in Galcliff
Recover the final dragon gem, and summon a dragon
Collect all the 4 skulls in Thornvik
Liberate the farm, still, lumber mill, and the quarry in Isaholm
Execute 100 sneaky kills
Use the ultimate fire attack to kill a Legion
Glacially wipe out a Legion using the ultimate ice attack
Send a Legion back to the underworld in the most lightning-tastic way imaginable