In order to unlock the achievement you must perform the kill, knocking out the enemy will NOT count. Therefor recommended for High Chaos run
Just to give some advice I found it easier to either jump or fall towards my opponant then rather than spamming
just wait for the option to pop on screen and as soon as it does hit that RT button.
Alas Domino does not work so this must be 10 individual drop assassinations.