Even a human brain has redundancy, i.e. parts of brain can take on roles of each other, at least to a degree. We have two kidneys and can live with one, same goes for anything else, pretty much, we have two of. transition from kidney 1 to kidney 2 is seemless...not sure why brain would be that different. maybe memory loss and eventually - high cognitive function. But not easily...
Now imagine you redesigned a human for maximum survivability. place all vitals in chest cavity and make ribs stronger. Feed 'em special goo, so that digestive system can be vastly simplified and the track shortened shortened. The remaining space can house the (second) brain, another liver, a third lung, second spleen... Any docs in here? What if there is no need to digest anything - just transfer from liquid to blood...No solid waste, either...How much room would be saved?
The extra space is packed with hard muscle that surrounds and compartmentalizes the extra organs. Compartments mean that bleeding is limited and fragments/bullets are less likely to damage multiple organs.
What else...Eyes - add a few more, some fixed around the head, a couple of flexible stocks. Expand the range of visible frequencies a little into infrared. Ears - screw it, two are enough, maybe three, but make 'em movable like those of a deer. Add hearing in frequencies bats use. In a pinch can be used for navigation. A couple of noses, two at the most - shooting by smell is not reliable, and two are plenty for tracking.
The head no longer houses the (only) brain. No brain - smaller head. At least thicker skull. Prefered "forward" is no longer needed. Periferal eyes + two on stocks guarantee that even with all eyes on one side burned off, the two stocks can still provide depth perception and allow to attack (shoot/strike/persue precisely) in the direction.
Speaking of direction. Three legs, at least 3-4 arms. 3-prong system is way more stable for shooting and even a complete loss of a leg "reduces" the creature to human mobility. 4 arms = 2 two-hand weapons. With full set of eyes - fixed perimeter eyes + two on stocks = depth perception in two directions at once, so aimed shooting in two directions at once. Hands: one thumb? Weak! two thumbs, three opposed fingers. I'd prefer all joints (at least sholders and elbows) to be omnidirectional. If that is too difficult muscle-wise, at least full range of motion in two directions - very double jointed. Makes hand-to-hand extra fun.
So, by my reckoning, with one leg and two arms blown off, half the torso a mess from a frag grenade or a rifle salvo, and side of the head burned to crisp - what is left will be equivalent to a pissed off PHALANX soldier.
how's that for ET?