From what I understand, most soldiers in war don't die because of guns. Explosives such as landmines or bombs kill the majority. The reason seems to be that the shockwave of an explosion more or less bypasses the Body Armor.
So really, if you want fewer casualties, having an effective way to guard against mines is the way to go.
In terms of being shot, composite body armor does the trick pretty well. But here, we have a different problem. The round doesn't have to pierce the armor. All of the energy still unloads on your body and can break your bones, leading to massive internal damage.
So yeah, you could probably create some sort of composite body armor that stops a .50 cal, but the person would die anyway from the bullet's kinetic energy. You would have to somehow redirect that energy.
Something that could do the trick is using body armor that converts kinetic energy into heat. This way, your organs wouldn't get destroyed by the kinetic energy. If the body armor is heat resistant as well, you could have effective protection against high cal guns. But, I have no idea how one would go about building something like that. It wouldn't be cheap.
Using implants as armor is a pretty dumb idea too, mainly because the bullet will be stuck in your body (which is bad).
You could try your hands on genetic modification as well. If you can create some form of rapid healing ability, most wounds wouldn't be too hard to deal with.
Until now, I only talked about Passive Protection, meaning armor that will get hit. Active Protection is not out of the question. Tanks, for example, use Active Protection by launching small explosives at an incoming round, deflecting it in the process. Something like this could also work for Body Armor.
Another type would be a gun that is mounted on your back that's used to fire at incoming rounds.
So what would be the best solution?
Energy-converting body armor is my guess. But, as with all converting, some of the energy will remain as kinetic energy. So, big cal guns will still hurt. Not to mention that the enemy could launch the bullets faster to the point where the armor just melts.
You could / should combine that with some sort of active protection though. The gun on the back seems cool enough for that.
Needless to say, such armor would be quite expensive, so I'd imagine only Special Forces could use it.