For this Achievement you have to kill 5 giant creatures in Fallout. Giant creatures are Behemoth and Mireluck Queen Enemies. During the Story you can kill 1 or 2 giant creatures. In the world of Fallout 4 you will find many more.
The Achievement is still glitched for some user. Here are 2 hints for you, kill all 5 giant creatures in one gamingsession and make sure you have always the killing blow!
I show you in video below 10 locations with giant creatures.
*** Spoiler - click to reveal ***00:09 - Behemoth
01:03 - Behemoth
01:45 - Behemoth
02:28 - Behemoth
03:28 - Mirelurk Queen
04:26 - Behemoth / Swan
05:19 - Behemoth
06:01 - Mirelurk Queen
06:32 - Mirelurk Queen
07:30 - Behemoth