Devil May Cry 4

Heaven and hell is a good way to obain it if you dont mind grinding for a couple of hours. a good place is mission 11 with nero and the place where you have to snatch an enemy onto the presure pad. dont, just stand next to the gate and keep firing into the crowd they keep on spawning and you can rack up kills.
Also you could just keep going through bloody palace which is a lot more fun than grinding and you can work towards the proud souls achievment while you are there.
also there isn't any visable counter (not any that i have seen) so you will have to keep killing
and then finish the mission so the kills are saved and hope it pops up at the end of the mission.

I’ve just finished this the other day and thought I’d share my method for tracking the kills done. Here's a way to very roughly keep track of your progress using the L11 Heaven and Hell method described above by ITS 0vER 9000.
- Take note of your red orb count at the start of the session.
- Kill for as long as you can be bothered to do and check your orb count again.
- Kills done during that time is approximately (new_count - old_count) / 16.5

This is because the first scarecrow to spawn gives you 13 orbs, the second 20... so 33 per pair so 16.5 per scarecrow on average.
This is only very rough as it makes the assumption that you kill equal amount of scarecrow 1 and scarecrow 2.

Using this logic if you had 0 kills when starting to boost this (very unlikely, but need to make some assumptions to make the maths work ;-) ), and a red orb count of 0, you would need to get to around 165,000 red orbs doing just those 2 kills to get to 10,000 kills total.
I got mine at around 120,000 orbs (over several sessions) having completed several run throughs on various modes. Don't forget to finish the level for the kills to count.

Hope this helps others who are finding this grind in the dark a royal pain to keep motivated with.

In addition to the previous solution, another good place to rack up kills is Mission 6 when you are battling Agnus in the electrified floor room. The swords keep spawning. My tip is if you have beaten the game (hopefully you have) use Super Nero so that you have unlimited Devil Trigger. Then just sit there and kill the swords as long as you like. You may hit Agnus's window a few times but this isn't Heaven and Hell. You probably want to go on the easiest difficulty just because you can kill the enemies easier and if you get damaged, it won't kill you. This achievement took FOREVER to get so just keep killing enemies. I had beaten the game on every difficulty and went through the Palace and still wasn't even close.

The best way as already mentioned, is heaven or hell mode in chapter 11, where you need to snatch enemy on pressure pad.Just for the record, i putted already 98 hours to the game, have beaten all the modes, and I still grinded for 3 hours.This strategy was best for me, because you can simply tap x button, and you can watch tv or do something else, you dont need to actually watch this tedious process:D So good luck to you all, its not hard, its just boring, but just tap x while watching some movies like i did and youll get your tasty 50gs ;)