Not a Paleontologist, but if you'd like an engineer's perspective:

So I'm thinking you're looking at the UMissouri paper regarding the strength of the skull. The stiffness they're referring to is irrelevant to your question.

One trouble our shot t rex has is, the skull is very skeletonized (hollow). The holes in it are huge and the skull is thin in many places.

Basically, in my opinion it's gonna come down where/ how it's shot even more than how big the gun is.

Of course, the thing is also huge, and all it needs to do is to not have a bullet enter the brain cavity with a lot of energy.

I entertained the notion of spending a couple hours doing some explicit dynamics simulations on this, but sleep's gonna hafta take priority :)

Yep! Turns out you're correct (says Google). Lasers do follow the inverse square law. https://www.quora.com/Is-the-light-from-lasers-reduced-by-the-inverse-square-law-as-distance-grows-similar-to-other-light-sources

How powerful the transmitter will need to be is also a function of the gain of the antenna. In this case, the spread angle of the laser

Quick correction here: transmission power falls off per the inverse square law only given an idealized isotropic antenna. Focused beams using, for instance, lasers, do not experience the same losses.

But yeah, it's cuz it's far.