I’d use the knife to cut branches and turn them into spears. Carve them really nice and pointy. I’d poop on the ground and smear the tips of my spears in my own feces. I’d find a way to sneak up on it while it is eating or sleeping og doing whatever. Throw my spears as hard as possible and as many of them as needed to make sure at least some of them hit the target. Then wait.

My biggest problem is that I am as atheltic as a potato, so I’d probably not be able to throw it far enough and hard enough to penetrate the skin. Depending, of course, on where thr heck I am positioned during the attack. If I am somehow able to be perched in a tree or on a tall rock before attacking, then maybe I am at an advantage. I could be supliment the spears with blow darts, also covered in poop, if I have the time to develop and use that tool properly. Ideally, I would like a bow an arrow, but I dont believe I’d have the skill and time to make a more advanced tool like a bow. One month isn’t a very long time, after all.

Since T-rexes more than likely had feathers instead of the scaly skin we associate them with, there is a good chance, the spear/blow dart would be able to penetrate the skin - if my aim, strength and speed wasn’t total shit.

I would also have to do all of this within the first week and attack it repeatedly to make sure it got infected beyond what its immune system could repair by itself. I have no idea how long it would take for a T-rex to die of sepsis, but the earlier it is hit, the better.

When you are smaller than your prey, you have to think of how smaller creature becomes a threat to bigger creatures. Poison is almost always the answer. The only poison we have ready avaiable, is our feces.

More than likely, I would totally fail in a real scenario and stay in my indestructable hut the entire time.