Here is the full list of all 13 Cows VS Vikings achievements.
Put out 50 burning towers
Upgrade 15 towers in a single battle
Disable 200 ninja cows
Use Thor's Hammer succesfully for 50 times
Disable 25 cows with artillery
Disable 25 cows by freezers
Disable 25 cows by wizards
Earn 5 barrels of milky Milk
Disable 25 cows by archers
Build 4 kinds of towers in a single battle
Disable your first cow
Put out the same burning tower 10 times
Sell 40 towers