1.A simple and easy way is to use poison.
2.Use hidden blades to assassinate them.
3.Easiest way in battles is to using fists by dodging their 1st hit then keep hitting until they die.
4.Use a smoke bomb then assassinate them.
5.Use heavy weapons.
6.Use the hidden gun.
Tip: if they try to shoot you with their pistols then hit them before they do.