Note: This cheat needs to be reentered after each loading screen. Any after-effects, bugs, or glitches that may corrupt your game are currently not known at this time. Saving your game with this cheat on may result in it being on permanently.
On the PDA's faction screen, enter UP, DOWN, UP, DOWN, LEFT, RIGHT, LEFT, RIGHT on the D-Pad. You should hear a click that signifies the code is active. The merc will be invulnerable to all sources of damage (you may have to Medivac if you fall into deep water). This also applies to the current vehicle the merc happens to be operating at the time.