You have to enter the following codes ingame (while Lara is free). Once you enter a code correctly, a chime sounds.
PC
Bulletproof Lara: Hold Target Lock and press Jump, Fire, Interact, Fire, Grapple, Sprint.
One-Shot Kill: Hold Target Lock and press Interact, Jump, Interact, Grapple, Sprint, Crouch.
Show Enemy Health Bar: Hold Target Lock and press Grapple, Crouch, Jump, Sprint, Fire, Interact
PlayStation 3
Bulletproof Lara: Hold R1 and press X, L1, Triangle, L1, Square, R2.
One-Shot Kill: Hold R1 and press Triangle, X, Triangle, Square, R2, Circle.
Show Enemy Health Bar: Hold R1 and press Square, Circle, X, R2, L1, Triangle.
XBOX 360
Bulletproof Lara: Hold LT and press A, RT, Y, RT, X, LB.
One-Shot Kill: Hold LT and press Y, A, Y, X, LB, B.
Show Enemy Health Bar: Hold LT and press X, B, A, LB, RT, Y.
PlayStation 2
Bulletproof Lara: Hold R1 and press X, L1, Triangle, L1, Square, R2.
One-Shot Kill: Hold R1 and press Triangle, X, Triangle, Square, R2, Circle.
Show Enemy Health Bar: Hold R1 and press Square, Circle, X, R2, L1, Triangle.
Special thanks to laracroftswest