The Witcher 3: Wild Hunt

Since non hostile animals (cows, pigs, goats, sheeps) count towards this archivement, go to the far north east.

There, shoot the cows and and sheeps and meditate to make them respawn.

• Go to the Devils Pit and lure the enemies up to the higher platforms. Then use Aard to kick them off. After you killed all enemies, leave the pit and meditate for 48 hours or quick travel to another map. Then they should respawn and you can do it again.
• This is easier to do with higher Aard intensity

• Make sure the opponent's health is low enough to freeze them and do a one shot kill. Otherwise, they'll thaw out and you must repeat it.
• Use only standard bolts. Ones like split bolts don't appear to work towards the achievement.
• Using freeze enhancements (like Piercing Cold) don't appear to count towards the achievement.
• While the wording says "opponent" it appears to work against non-hostile creatures as well, like deer.

• Go to the far north east in velen as seen on the picture.
• Cows are on the northern site of the farm. You can kill them, meditate and repeat until you've reached the 20 kills.
• Use the steel sword to cill them. Other methods don't count towards the archievement.
• Sometimes when arriving at the place. Cows didn't spawn yet. Just meditate for an hour and they will spawn.

• Equip a bomb to quick throw.
• Go to any enemy and start with the counter attack (parry in the perfect moment).

• Ignite an enemy with Igni, then throw the Dragon's Dream bomb at him.