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