1 Longneck Rifle.
Around 200 Darts incase. Depends on lvl
4 Stone gates.
1 Large bear trap.
Place 3 gates next to the large bear trap.
Get the rex attention and run past the bear trap.
Until the rex gets stuck in the trap.
Then place 1 gate behind the rex don't get to close.
Then shoot rex until it knocks out.
This also works with spino