Fallout 4

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.

A helpful tip for later on down the road:

If you unlock the Settlement "Murkwater Construction Site" A Mirelurk Queen will spawn there. I don't know if it will be there everytime that you fast travel there, but of all the times that I have, there has been one there.


Location:

*EDIT* - You may have to wait in game days for the queen to re-spawn. [Thank you to: lLeXXuzl for the heads up] Travel to different settlement [Sanctuary Hills, Prydwen, so on] and sleep for a few days. (I've noticed sleeping can go allot faster if you're in a pace with less stuff, and you've just loaded up the game.)

*EDIT** I appologize for the previous image I had for this solution before this edit, I changed it to just lead to an Imgur page.

Spawning these enemies in did not unlock the achievement for me so I had to travel to 5 locations with giant enemies, but then the coc command didn't want to work for me either so I unlocked all map points and fast traveled to each one the old fashion way. Once at the location kill the giant creature (behemoth or mirelurk queen depending on location).

Press " ~ " to open the console
tgm - God Mode
player.placeatme 0021a53e - Big boy
tmm 1 - Unlock all map points and fast travel

If you want to try spawning in behemoths use: player.placeatme 000BB7DD 5 if it works let me know in the comments, might just be buggy.

If you want to try the coc command:

coc swanspondext ("swan" named behemoth)
coc POIBryanB01 (mirelurk queen)
coc SuffolkCountycharterschoolext (behemoth abit to the northwest, use VATS to spot it easier)
coc WaldenPondext (the behemoth is just to the south among some ruined buses) (**Lvl 15 requirement** player.setlevel 15 thanks to VidGamiac for pointing this out!)
coc SpectacleIslandext (turn on power in the boat on the south shore to spawn a mirelurk queen)

For fast travel:

Swans pond is located right in the downtown area, abit northeast of diamond city, the icon is a swan but its kind of surrounded by house icons. There is a behemoth named swan here.

Murkwater Construction SIte is in the swampy area on the southeast of the map. A mirelurk queen resides here.

Suffolk County Charter School is just north of the last location, once you spawn in travel to the northwest using the VATS button to spot a behemoth.

Walden Pond is directly south of the Red Rocket Truck Stop. Once you travel here go abit south to some ruined buses where you will find a behemoth, use the VATS button to spot it quicker.(**Lvl 15 requirement** player.setlevel 15 thanks to VidGamiac for pointing this out!)

Spectacle Island is the large island on the southeast of the map, once here go to the southern shore and inside the boat is a breaker box you can activate, once you do a mirelurk queen will spawn.

** BONUS: An extra behemoth can be found at the super duper mart in case you can't get one of the above creatures to spawn.

coc SuperDuperMartExt

Thanks to Fightclxb for this bit!

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. I show you in my video below 10 locations with giant creatures.

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!

Another good hint is, use a power armor. With a power armor you have much more defense and you can longer fight without problems against the giant creatures.

This solution assumes that you know how to use the in-game console (accessed with ~ or @ and some other keybinds depending on your region) and that you have progressed past the opening of the game out of the vault and into Sanctuary. You can check the keybinds from the main menu if you need to refresh your memory.

Index:
1A. Overview
2B. Prep Console Commands
3C. Getting the "giant creature" kills
====
1A. Overview
-Some of the enemies at certain spots didn't seem to spawn for me, for one reason or another...
--IF this happens to you:
---Just skip ahead and go to a different area I have listed
---Try waiting a few days (by sitting in a chair) and going back to the location
---Checking the wiki for additional info.

-Console commands will give you all map markers, invincibility, weapons

-Use this interactive map at https://fallout4map.com/ to make pinpointing areas easier if you are having trouble

====
2B. Prep Console Commands

1. If you care about your current save you should make a new one as a backup

2. Enter the following console commands hitting enter after each one
tmm 01
tgm
player.additem 000e6b2e 500
player.additem 000BD56F 1
player.setlevel 15

3. Equip the Fat Man weapon

4. Make a new save so that you can load back with all the weapons and commands entered (in case you crash or something goes wrong)

====
3C. Getting the "giant creature" kills
-You are travel to the areas listed below one at a time, find the boss creature (not difficult or challenging) and kill them with the Fat Man mini-nuke weapon

-I will list the command you need to enter (in italics) and then under it additional information about where to find the creature

-Make a new save after each successful kill if you want to be extra safe

-If you are taking a long time to load into a new area (particularly coc swanspondext) just be patient, it WILL load. If it hangs forever then exit the game / Alt+F4 and retry.
---
CREATURE 1
coc POIBryanB01
-Go directly South and you should see a giant Mirelurk Queen near the ruined building

-Images
--Map:
--In-game:

-
CREATURE 2
coc swanspondext
-Go very slightly North East into the little fenced off area (towards the Swan/bird map marker) and a Super Mutant Behemoth called Swan should rise out of the water/mud

-Images
--Map:
--In-game:

-
CREATURE 3
coc SuffolkCountycharterschoolext
-Go just North West past the yellow bus into the little swamp land area with water and you should see a Super Mutant Behemoth

-Images
--Map:
--In-game:

-
CREATURE 4
coc spectacleislandext
-Go far south to the opposite tip of the island and you will find a crashed green boat semi sunk along the shoreline. On the first/ground level of that boat you can walk in and see a compartment above the blue control panel. Interact with this square object, activate the breaker and a Mirelurk Queen will spawn outside the boat

-Images
--Activate the breaker:
--Map:
--In-game:

-
CREATURE 5
coc RoadsidePinesMotelExt
-Go far south-west towards the electrical tower in the distance and in that area you will find a bunch of metal pillars in a squared off section with several deathclaws and a Super Mutant Behemoth in the middle.

-Images
--Map:
--In-game:

====
The achievement should pop about 15-30 seconds after the final kill. If not you can restart your game and see if it pops right then or try from an earlier point.

Thanks to Dream Fiend for writing the solution above, as it helped me get the achievement

Feel free to republish, edit, or reproduce any parts of this walkthrough as you see fit

Make sure the combat difficulty is set to Very Easy to ensure the quickest kill.

You will find two giant creatures during the story. One is a Behomoth you find in the Brotherhood of Steel questline. Once you've defeated this Behemoth, make sure you join the Minute Men and defeat the ML queen. Then, head over to Swan's Lake and approach the white swan to reveal Swan (a Behemoth). It's located southwest of Goodneighbor and east of Hubris Comics in the main metropolis. After that, head south of Suffolk County Charter School to find an ML Queen at Murkwater Construction Site. On the east side of the map on Spectacle Island, you'll find another ML Queen. To access it, you need to turn the power back on at the boat on the southern end and the central hut with a communications tower. That should be your final one. If you haven't joined the Minutemen yet, there's another ML Queen found deep within the State House.