Unsouled

Best place to grind this out is about a bit over halfway through Act 1. Once you make it to a statue with one of the ninja enemies close by to the right, head to the field and go all the way down to the left. Let one of the enemies push you off - by the achievement’s words - the ledge and most of the enemies there will be conveniently on the edge of the “ledge” itself. Use Soul Burst ( then ) to fling them into the abyss; finish the remaining enemies off from there (if there’s a decent amount of enemies left near the ledge use Soul Burst again). Afterwards, go to the bridge, cross it, and hold so that you can destroy it and get all the enemies on it to fall to their deaths. Confirm its destruction once some of the enemies from the ambush are in your sight, at that point either all of them or all but one will be on it. From here, go to the menu, press “Quick Restart”, and repeat until this pops. This is probably also the best method for grinding out ‘Shura’ if you’re still short of kills after taking care of the rest of the list; as of the making of this guide I’m about a third of the way there.
EDIT: You’re probably going to get ‘Shura’ while grinding out the enemy Passive Souls for ‘Soul Taker’ unless if you get very lucky with the drop rates, I got 2,347 more kills then I needed for ‘Shura’ by the time I unlocked ‘Soul Taker’.

As a PSA for those that want to track their progress on this: Once you’ve finished the game, you’ll be able to see the amount of fall kills you have done via a statistics menu.