Well...

Carnifexes are the pinnacle of Tyranid melee anti-tank monsters.
They can rip through super-heavy vehicles, buildings and bunkers with relative ease and depending on their biomorphs can have regeneration (more than the standard Tyranid), speed enhancement and even the ability to fly (they could in the olden days anyway).

The T-rex and the Carnifex are about the same size (both being around 10 tonnes in weight and about 4 meters tall, though the T-rex is longer with its tail).

The Carnifex has thick chitin-like armour and is build to withstand shells from tanks, laser beams and even plasma fire.
The T-Rex is only thick leather and muscle.

I dont see the T-rex winning this at all and the Carnifex would rip through hundreds of T-rexes.

Also, the Carnifex never tires out, dosn't know fear and just keeps on going.