Freehub body will not undo
On a lot of the 12mm ones, the part that has the 12mm wrench flats is essentially a nut threaded on to a protruding thread off the freehub body. In that case you're wanting to turn the wrench clockwise when viewed from the drive side or counter/anti-clockwise from the non-drive side. 

They're one of the higher torque things on bikes and sometimes threadlocker is applied at the factory as well, so taking a lot of force to break free isn't necessarily a red flag as long as you're being sensitive that it doesn't just feel like you're turning the wrong direction.