Doc J - or move them off a sniping position... I was thinking, if you can't kill a sniper, at least shove them off the roof, giving troups time to close in...
Sirg - alien without a brain was a joke, really. But they may very well have no single vulunrable brain. Place it in the chest cavity and it becomes much more resiliant. if Ortnoks are bread for war, that is not an unexpected solution.
* WARNING! Science stuff according to Dan *
Effects of impact...
Projectile (say, .50 cal bullet): It can either go thourgh a target, making a hole (think NATO .223 round on belly) but retaining a lot of it's energy or be stopped, not make a hole, but then you have an inelastic collision, i.e. almost all momentum and energy is transfered to the target (think .357 magnum or a .45 - stopping power, baby!). If alien armor protects the alien, we are in case B, no hole, momentum absorbed. Now, if alien was a solid chunk of diamond, straight conservation of momentum: m_bullet * v_bullet = M_target * V_target. If target is 10 times heavier than the bullet, it will acquire the velocity 1/10th that of a bullet, in same direction.
If the armor does not absorb the bullet but only behaves like a super-hard shell, that is what we are dealing with. Alien flies back. Now, soft alien tissue must be in contact with the armor. Which means that a shockwave will propagate from point of impat on in. This is what happens these days with a human wearing armor - huge bruises, broken ribs, in extreme cases - ruptured organs, even if there is no hole. The armor destributes the imact. Instead of a piercing needle, the hit is like with a sledgehammer or a cement bag. The alien armor may try to do a few things at once - "eat" some of the transfered energy by having a plate the shatters - thus taking some of the energy. May also have a gel-like layer, which absorbs momentum and the shockwave gets redirected throughout the armor itself, not the tissue. So, the "hard shell - shatter plate - soft absorbing lining - hard shell" design might help to reduce both the point impact and tissue damage. but still - momentum is conserved. Even if the organs are not liquified, significant enough momentum will shove the alien back.
Explotion: the flying pieces, for the most part, behave as bullets and we need not look at them. The shockwave, however, is a different story. In a nuke, btw, it is the shockwave, not the heat nor radiation that do the most immediate damage. It is the good old compressed air. The shockwave, in short, is the area of very high pressure traveling fast away from the center. The farther from center, the weaker the wave (falls as 1 / distance^3). This one need not be destributed by the armor - it affects every surface that the wavefront touches. It just transfers momentum, a solid wall of air. That feels like a cocrete wall. Again, the super-duper armor may keep the organs from liquifying, but being tosseed about is prety much mandatory.