You want a monster of a species that's tough to eradicate? Look at yourself in the mirror.
In both your previous question and in this one you seem to be looking at it wrong. If you want you monster rex to be a bigger threat, you ought to look at the approach they took in the 2 jurassic world movies (spoilers ahead).
In the first movie they made a hybrid called the Indominus rex, a creature which was powerful, decently bulky and, more importantly, smart. It was able to escape containment, ambush humans and overpower a T-rex, and yet, despite having longer arms, it was incapable of doing anything when the humans entered a building, because it was too big. It's size also made it a much bigger target, which is one critical problem with humans: being bigger means you're tougher to kill, but also that you're not as mobile, which means that if I have enough pointy sticks to throw at you, I can just lure you into a big hole I dug or into another place to further hinder how much you can move around and just throw sticks at you until you die. Better yet: I can use your own weight against you by making a trap that can sustain my weight, but not yours, so that it goes unnoticed until you're on top of it.
So what happened in the second movie? Despite all the wrong ideas about why one would want a dinosaur in the military when you have anti tank rifles and drones, they did think something right: making the indominus rex smaller. That way it can access places where humans would hide from it, it can be faster and more mobile, it's a smaller target and one of the most important of all: it needs less food, and that is important specifically because a bigger and better brain requires a lot more nutrients to maintain itself at full capacity than a brain like that of a normal T-rex, and if you have a big body, while it can be said you'll need less food in proportion to your size when compared to a smaller animal with a faster metabolic rate, you still need more food in absolute numbers, which means a bigger body takes more food to maintain it, meaning adding a more powerful brain would further increase this already large demand, and as we already know that even without a human brain the T-rex already needed a bunch of food.
So how did it go in the movie: they made a smaller indominus with some different abilities but a few important traits to mind:
it was smaller and faster, but still larger than a human and, not being as massive as a larger animal would, it could deal with some added weight from tougher skin.
it was pretty smart, and understood well enough concepts like luring in prey by pretending to be knocked out or dead.
It could afford to have the longer arms you wanted in your previous question, because in here the smaller size naturally meant less space for jaw muscles, and thus the ability to use your forelimbs to help with climbing, grasping and pinning down prey make them more than worth the cost.
The main problem with this animal in the movie was that it was naturally bound by the laws of movie antagonists, meaning it worked much less like an animal hunting and more like a cartoon villain, but it's effectiveness is not to be ignored.
Basically, to get the horrifying monsters you want, your T-rexes should reach their top size at the juvenile stage, in which they're already larger than a human and powerful enough to overpower one, but the fact that they're far from being as large and heavy as an adult would make them much more mobile, much like it's assumed to have actually happened in the past, in which juveniles were assumed to hunt weaker, but faster prey, as they were more well built towards speed, while the adults hunted the bulkier, slower prey that required more raw power to be taken down; a difference which allowed for less competition between the adults and juveniles.
(image taken from saurian wiki)
Another major difference is slight tougher skin and longer arms. Since our T-Rex is smaller than a normal adult would be and has less powerful Jaws, additional help with pinning down and attacking prey is a good way to go. Modifications to allow for a wrist that isn't eternally locked in a bird like position is also more than welcome here. They'd still be smaller than the legs and would require a heavier tail, but would be more helpful than in the case of a fully gown adult.
Finally, the brain: your monster t-rexes will need essentially mammalian level brains, ideally almost as good as ours. Why? Because they're designed to be able to survive against humans in the long term, and the only strategies so far that were truly successful were living in locations we can't possibly go and never leaving, appealing to other humans to help you escape extinction or outsmarting us, with the last one being the hardest to pull out. Furthermore, we'll have your monsters gain durability by being less antissocial. We're making them pack hunters, because the smaller size and bigger brain means both that they're now capable of getting together and making coordinated attacks, but also that now, since they need less food, they can afford to hunt together without having the whole group starving. Add in some traits like a tougher more leathery skin and some osteroderms and you got a beast.
Summing up:to make your monster T-Rexes we'll make it so:
1-they're smaller, meaning they're more quick on their feet, are better at both pursuing and escaping at high speeds and so they need a lot less food to survive, meaning they can afford to be more social and that the environment can sustain more of them at a certain period. While this technically makes them less capable of tanking attacks, it increases their chances of surviving, meaning they can endure more damage in the long run than a more well armored T-Rex that wasn't as able to run away, because your wounds can only heal if you're not dead.
2-they're smarter, because we're proof that intelligence is by far the best strategy, and if their strong bodies come packed with a brain almost as capable as ours, they'll already be much more dangerous on their own than a fully grown but dumber adult would be. They're smart enough to track you and to trick you into a trap and they're small enough to be able to follow you almost anywhere you try to go to hide from them. They can also use this intelligence to avoid larger groups of people and isolate them in more manageable groups. Intelligence is by far the best weapon to make them more capable of surviving and more dangerous foes. They should be at least intelligent enough to understand the concept of using tools, so that they can potentially use things like shields, which can temporarily boost their durability and be ditched should they need to run away.
3- they're pack animals, because now instead of one big slow monster that will stop being a threat once you kill it, you have 6-10 smaller, faster and smarter monsters which can access your buildings, and if you kill one you're still 5-9 dead monsters away from being truly safe. While the individuals aren't as tough, the pack as a whole is much stronger, plus you'll need to kill all of them to end the pack. A pack is also much more capable at making traps and cornering an isolated human, and since they're faster, the human will be out of their element, since their ability to walk long distances will be worthless when they're caught in an ambush. Remember that even today an isolated human with a rifle can still get killed by a pack of wolves if they're not careful.
4- they're only slight tougher than a normal juvenile, because too much armor would start to hinder their speed and mobility, and the fact that they're pack hunters already makes them pretty tough simply because unless you're in a group as big as or larger than them, you won't be able to focus on a single target without risking being attacked by another member of the pack, which means your chances of attacking effectively and surviving go down. If you have to choose a pack that's tougher and one that's more mobile to fight humans, go for a more mobile one.
5- they will Finally get more proportional arms, but more importantly, they'll get more articulated and dexterous hands. The fact that t-rexes managed to reach adulthood should by default mean they weren't exactly incompetent hunters while in their juvenile phase, but it doesn't hurt to have arms capable of grasping. After all, humans have them, and a good amount of all of our weapons, tools and overall technology require dexterous forelimbs to be used. They might not be able to throw a spear or fight as well with a sword, but they might still be able to grab a human, pull a horse by its lead and operate a modem shopping cart. Now, what does having longer arms capable of grasping have to do with defense? Well you can hold a shield, can't you? And so can a chimpanzee, so remember to try to make your dinos as smart as those if you can.
If your monster rexes indeed follow all of these, you'll get a group of extremely efficient, scary and durable predators, because your brains suddenly aren't as much of an advantage as they'd be with a mindless beast, your weapons become less effective because you have much more than one target to focus on (although they'll still cause a fair amount of damage should they hit), your enemy can use some of your weapons and defenses against you and despite being only a bit dumber than you, they're physically much more capable. You're built to walk long distances and use that to exhaust prey, but they're just as good as tracking, but they won't need a horse to run fast, or to outrun you while you dash to warm the rest of the village about the monsters in whose trap you've just fallen while out to hunt in the forest nearby.
It may sound counter intuitive, but 5 normal hunters with the strength of one and durability of one can be harder to kill and better at hunting than a single hunter with the strength of 5 and the durability of 4, especially if the one big hunter is dumber.
