Jurassic World Evolution 2

This can now be done in Sandbox mode in Creative mode since all dinosaurs are now unlocked.

Build a Lagoon and attach a Lagoon Hatchery.
Build a path to an Arrival Point to activate the Lagoon.

Go to the Lagoon Hatchery and incubate a Mosasaurus.

The achievement will unlock after it is released.

As far as I'm aware the only way to achieve this is in the "Jurassic World-Chaos Theory" (Correct me if I'm wrong).

If that's NOT the only place, it is certainly the EASIEST as I've played the entire Campaign, all Chaos Theories, and most challenges, and this is the only one I can recall coming across this Dinosaur.

It is Lagoon Oriented, and around 2/3 of the way through your progression of the park. This "Theory"'s difficulty rating is the maximum, and it can definitely feel like it if playing sloppy. For example, even when i thought l learned the game and was good enough to attempt it, I hand't reached this stage of the mission until around 6 hours of playtime. Stopping and doing some challenges on Medium to gain experience, and I ended up reaching that point in around, or less than, 2 hours.

In short, even if you aren't the best player, you don't even need to fully complete this mission in its entirety to unlock the achievement. As long as you can get around 2/3 of the way through and complete the incubation and release of the Mosasaurus