The first question I thought of was how long the T-Rex would last.
For the purposes of the exercise, assume that the city government is paralyzed and the State and Feds are tangled in legal questions about endangered species, so the citizens are on their own.
NY has some issues with citizens owning guns (the Sullivan Act had more to do with politicians protecting their pet street gang supporters than public safety), and something big enough to stop an elephant isn't found in every local Walmart. So, what do they do?
If the net and cellphones are still working, put together a tracking system. Malicious folk will try to subvert it with fake sightings, but I think a majority rule evaluation and attentive administrators will be able to keep it real.
Then pick weapons.
A half dozen guys with 30-06's from a 3'rd floor window might do enough damage in a half a minute to make the T-Rex wish it hadn't chased the Time Traveler.
Get large pile of meat from a butcher and poison it, and have some brave soul haul it to a street ahead of the beast. Then wait and hope.
A dozen lads with Molotov cocktails and motorbikes.
Hijack a dump truck and run it in reverse to break his leg. Tricky, because he can probably turn fast enough to get that tail in motion, but ...
Yes, I'm punchy. Why do you ask?