The Witcher 2: Assassins of Kings

A great and early place to do this is in the prolouge when you're following the king around the castle right after exiting the giant wooden transport tower and there's lots of fighting going on around you there are many distracted enemies fighting other allies, this is a perfect time to attack any preocupied enemies from behind for extra damage, most of the enemies during this part ignored me for the most part as well so there shouldn't be any problem as to getting hit, if you don't get it on the groups before and during the ballista sequence don't worry as there are more afterwards

you can save and reload and still have your previous tracking for this achievement intact, so if you kill a few then run out of bad guys just save and your Perfectionist progress will be saved too, example you kill 6 without loosing Vita, save, keep playing then get hit, simply reload your save and you will still have 6 kills without loosing Vita tracked and will only need 4 more to pop the cheevo
thanks to HaZiiHaLo for this tip

This is relatively simple once you find the right area to kill 10 enemies. At the start of the game once you storm the castle you have to use the Ballista, this i the perfect area to earn 10 kills as there are fewer enemies and they come at you in small waves.

Once you get down off the wall, kill the enemies on the stairs to your right. These guys just cause you problems if you don't. Now from here try to kill everyone else in the area without being hit. In the video below I took a small amount of damage just before I got to the Ballista, so the counter starts with the knight and soldier next to the cross bow. Every time you prime the Ballista two enemies will appear. Kill all the enemies without being hit and use your YRDEN Trap Hex to slow down the knights with shields.

If you manage to kill 7 enemies shields this area without taking damage, this leaves you to kill 3 more in the tower. The enemies in the tower won't be paying attention to you, so they're easy kills. But this is a good place to earn the extra kills you may need as there are quite a few soldiers up here to fight.

I attempted this achievement a few times with the guides given; after a few tries, I gave up and told myself I'd finish this off playing an easier difficulty. I however got this achievement accidentally while playing on Dark mode and I'd like to share how. Spoiler free.

When you arrive in Flotsam there are tons of missions that need to be activated. One of them is a quest called "Malena", you can activate this quest in small town outside of Flotsam called Lobinden.

This quest leads you to a cave. In this cave are near 20 Nekkers. Before you enter this cave you need to have a few things: upgraded witcher sword, and about 10 grapeshot bombs. The grapeshot bomb diagram can be bought from Cedric in Lobinden, then you can make the grapeshot bombs with your alchemy skills.

Also this is very important: You need to have a knowledge ability of 3/3 when fighting Nekkers. If you don't, you won't do as much damage to them. To check this simply go to Character tab>Attributes>Knowledge

Now for the easy part. All the nekkers are randomly standing around in the cave. Activate your "Quen" ability and run through them so they start following you and remember it's alright if they damage you BEFORE you start killing them. Once most of them are following you and they begin to cluster together start throwing grapeshot bombs at them every few seconds, but be sure to keep running and dodging as to not get hit. It took me about 30 seconds to kill 10 of them on the hardest difficulty. Use the silver witcher sword if they get to close.

3/14/13 Edit. Fixed grammatical errors and some word re-arranging.

Just to piggyback gamerscorhunta:

The quest he mentions, Malena, is the perfect place to get Perfectionist. Right before you enter the cave, make sure to activate the circle of power with your medalion, drink your preferred potion to increase blade damage, as well as regenerate vigor, and optionally, coat your silver sword with oil. Save at this point, so you don't have to do it all over if you fail, then enter the cave and hop down the as gamerscorhunta mentions. My preferred method was equipping bombs (summa, or something like that is what I used and also netted me, unexectedly, the Torn Asunder achievement) and using the Quen sign, and using the most handy dodge roll technique. As soon as the nekkers spawn, I activate Quen and run and quickly finish off a couple, dodging rolling back to reactive Quen (do this anytime you see the blue shimmer surrounding you start to fade, or if you hear the distinct sound that is made when an enemy has destroyed your forcefield). Keep this up and when you are mobbed up on, use your bombs which will inflict some damage, but most importantly, give you time to dodgeroll to a safer distance and reequip Quen if you need. Just continue to dodgeroll in and slash at the nekkers and dodgeroll out. Just as a precaution I always used quick strikes (A buttom) so as not to slow down with the heavy strike animation, but nekkers not having much health and having powered up your blade, quick strikes go right through them. If you feel the need to save during combat, do so, but just being cautious, there shouldn't be any need....just remember Quen and dodgeroll are your friends!