NieR

Insanely easy. In the very, very, very beginning of the game, you're in a courtyard-like area with enemies that keep spawning and spawning and spawning. With the game on easy, you almost never get hit, so just keep slashing until you're at 50 hits. I got it without even trying.

The first good spot I found to get this early on was when I was directed by the main story to go to the Junk Heap. In the north plains as you're going down into the lower area by the lake, there should be TONS of little tiny shades. Wait above them or run around among them and let them gather up tightly- there should be like 30 of the things. Then just start laying about with your sword. If you get tagged hit block as you are in mid-knockback at the right moment(left trigger) to recover out of the knockback early so you can keep hitting and not lose your combo. If you managed to gather them together tightly enough, although they did that by themselves both times I did it, you'll easily get 50 hits as you kill them. (I guess I should note that this was on normal... on easy they might die in less hits and not survive for 50 hits, but if that's the case that's your own fault for picking the path of the weak :p). It helps to equip your weakest sword if you are overleveled and killing them too fast. Either way, once you fight the reviving boar later on in the Shadowlord's castle, both combo achievments are cake. The thing is basically a target dummy once you incapacitate it the first time. You can just keep beating on it for as long as you like after that, although it does release very weak poison gas while it's trying to revive itself.

In the second half of the game you will eventually be given a quest by Emil's butler, "A Tale of the Study".

An unusual non-aggressive Shade has appeared in the study of the mansion (where you fought the tome). The shade has a strong barrier that reflects all magic and is impervious to all but your strongest weapons. You can contiguously attack it without it dying or retaliating against you and easily get both combo achievements in little over a minute.

To do so, equip a weak weapon like the Nameless Sword and order Kainé and Emil to stay back (3rd option). Then just swing away and keep an eye on it so if it moves you won't loose your rhythm.

Once you have this achievement and Combo Master in NIER, switch to your strongest weapon (probably the Phoenix Spear at this point) and finish it, then turn in the quest.