40 Nuka-World Creatures is not that much, it sounds like much but it isn't.
You have 5 Nuka-Mix recipes unlocked from the start and can get a few rather long lasting ones from areas with enemies that don't count towards the achievement.
My recommendation is to gather all and any Nuka-Colas you find and start off by doing the Galactic Zone or the Kiddie Kingdom, as both of these do not have any enemies that count towards the achievement.
The Galactic Zone also has three Nuka-Mix recipes with rather long buff durations. It is important that the ones that only restore HP or AP also count for the achievement, as long as you have the effect from a Nuka-Mix one it counts.
After doing that make a bunch of any Nuka-Mix flavors, the longer they last the easier it is.
The next step is doing either Dry Rock Gulch or The Bottling Plant as they have many creatures to kill for the achievement, most notably the Nukalurks in The Bottling Plant, Bloodworms and Ants in Dry Rock Gulch.
It is very important that you keep an eye on the buff.