Jurassic World Evolution

The other solutions actually weren't working for me, as I have a T Rex that ignores the Gallimimus altogether under normal circumstances. It is to the point that I now keep that T Rex housed alone with a herd of Gallimimuses. It's been ~5 hours now and the T Rex has not ever so much as attacked them. Even putting the T Rex in a super small pen alone with the Gallimimus will not work because the T Rex would just break out rather than eat the dang thing. So keep it in its normal pen.

So here's how I got it to hunt them: I just removed all other food sources and then waited in the enclosure. You can actually sit in the pen with the T Rex. If you're in photo mode the T Rex will not actually be able to knock your vehicle over, even though it will try to. No matter if the T Rex is attacking you, it will eventually eat. If all it has to eat is a Gallimimus, it will do so. Just stay in photo mode, and stay pointed at the T Rex. Take the photo when the T Rex charges at a Gallimimus. If you're not sure if it's hunting, just take the photo anyway and see what the text says, then cancel if it's not. The achievement should pop after you accept the photo of the hunt.

Build a paddock.
Put a T. Rex in it.
Put a Gallimimus in it.
Get in the Ranger Jeep and park up nearby.
Wait for the T. Rex to charge the Gallimimus and take the picture as it attacks.

It's pretty self explanatory, but a quick note here. I couldn't get this achievement by driving the jeep into the paddock - the T. Rex would lose interest in the Gallimimus and charge the jeep.

Park the jeep up outside the fence and take the picture as the Rex attacks the Gallimimus. If you're playing the game sensibly, you'll have a concrete paddock wall, which blocks you taking pictures through it. Solution here is downgrade a section of the fence so you can see through it. You can easily reupgrade it after.

In this video I will show you how to take a picture of a T-Rex hunting a gallimimus.