No need to worry about this one as you will definitely get this through natural play progression. Nevertheless this includes; Deathclaws, Feral Ghouls, Super Mutants, Mirelurks, any other irradiated wildlife and anything not human (which presumably means that Synths are included here too).
[Taken from the official Fallout 4 companion guide]
As a tip, the streets of Boston city are teeming with ghouls and super mutants (as well as raiders) and you can check how many you have killed in the STATS section of your Pip-Boy.