I have recently started playing the on-foot part of ODY and working the engineering and upgrade for weps and suits path.
My experience thus far is: Only killing someone while on foot (by any means ie guns, electroshock, fists) is the only way that the Mecenary rank advances. As I am only 25% into Mostly Defenceless, I am seeing a 2 kill for 1% rate of increase. I am presuming that there is a similar sliding scale like that for ship to ship combat, but as I am only 2nd rank, most of the NPCs I have killed are higher ranked than me, so 1 or better EXP per kill.
But I have also noted that while NPC kills while on-foot advances my Combat rank, nonViolent missions on foot (ie Covert Thief, Digital thief , Digital infiltration) and killing on-foot NPCs with my SRV or ship are also advancing my Combat Rank. Also that this rate of increase appear to be higher than I would expect fighting Ship to Ship.