It seems that you have to get the Killing Blow, so you are the one who kills it.
Also the achievement seems to be glitched, like many others, as people are not getting it even if they have killed 5 or more giants. (Mirelurk Queens and Behemoths)
Just found this video - not mine, but I think it's useful.
It's PS4, but the game should be the same.
Credit goes to HarryNinetyFour
There are more then just those in this video.
-At Swans Lake (north of the Combat Zone) there is Swan (Behemoth).
-You find a ML Queen during the Minute Men questline
-You find a Behemoth during the Brotherhood of Steel questline
-You find a ML Queen at Murkwater Construction Site (Thanks to B MAN LIVINLAUT and his solution)
-A ML Queen is on Spectacle Island after you turn the power back on
-A ML Queen is found in the State House
-There is a Behemoth south of Walden Pond
Thanks to all those people in the comments who pointed some things out.