You can do this in the level Lost Tunnels. After a few minutes into the level you will come across bandits. A few are sitting around a fire and a few others are on patrol and/or standing around.
This is were you can farm those kills!
Just look out for the patroling dudes and kill them first and then go around for the other bandits - don't forget to loot their bodies to get your throwing knives back!
Reload the last checkpoint to repeat the process until you get the trophy.
Good luck!