Sequence 4 Memory 2 is a popular one for some of the other achievements, and it works well for this one, too. Jacob starts on the roof of a building across the street from the distillery. There are a bunch of enemies around the front of the distillery. You can shoot a line across to the distillery roof and assassinate any of those guys from the zipline. Afterwards, zip up to the top of either building, and repeat. If you run out of guys to kill, just reload the checkpoint and start again. Also, dying isn't a problem because it'll just put you back to that same checkpoint.
This one will go a bit faster if you try to assassinate two targets each time from the zipline. Just watch for the groups of enemies walking around together.