2 0 0The 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.