NieR

Simple. Kill tons of sheep. Some of the animals in the northern plains are goats so take care not to waste time with them if you're just grinding for this. Find an area with a ton of sheep around, hit start and hit "right" to "orders"-"animal"->"call" and you will whistle and all the nearby sheep will come up to you. Then just use some magic like the fist smash in the middle of them to kill them all at once without having to chase them down... or you could just run around sniping them Dark Blast. They don't respawn, so after killing a good number of them just zone out and back in to the north plains and continue farming. I'd just suggest killing all the ones you walk by whenever you happen to cross the zone rather than grinding for this, because you'll be running across it a lot, and by level 7 you'll be 1-shotting them with single sword attack on Normal.

Eventually all the sheep will be replaced by goats completely and then shades as the story progresses, so keep an early save if you want to go back and farm this one. The exact point at which sheep will disappear is after you set in motion the events leading to part 2 of the game. Popola will ask you to retrieve some sort of moss from the chicken pen in the village. As soon as you touch that, a large sequence of events are set into motion and there will be no more sheep at all afterwards.

There is also, in the DLC, a totally WTF out of left field sheep massacring minigame in which a guard in the NW corner of the northern plains gives you. You are put into a special sequence where you are plopped down on the northern plains which is swarming with dozens of sheep and your objective is to ride your boar into as many as possible and when you collide with them they go flying 50 feet into the air and are instantly killed. You can do this quest at the very end of the game and as many times as you like, even after the sheep are gone in the main game, but I cannot confirm if killing them in this manner counts toward the achievement.

Right at the beginning of the game where you need to get mutton for Popula there will be tons of sheep. Once you get to the northern plains press start-->orders-->animal-->call. This will call all the animals around to to your location. Once there is a group of sheep together start swinging your sword. Once they run away from you simply call them again. They will all run back to you. You should be able to kill a sheep within 5 hits at the beginning of the game. And if a sheep seems like its taking a while to kill, its not a sheep. Its a goat. These do not count towards the achievement so there is really no point in killing them when grinding for this.

Go north of the main town and just kill sheep until you get the achievement. NOTE that this must be done within the first few hours of the game, since the sheep won't be around for the entire game.