But for some enemies, it's not entirely obvious where the head is. Or they don't have a head. Or their head isn't actually their weak point. For those cases, I have prepared this simple diagram.
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Brute - The head is a major weak point, but it's also a small target often being hidden behind their large, heavily armored, highly damage resistant crab claw. The Brute doesn't have any other weak spots, but their armor plates resist damage until they're broken off. So try to aim for unarmored areas like the knees, or next to the spine. When they charge past you, turn around and shoot them in the backside.
Cannibal - Some people seem confused about where exactly a Cannibal's head is. It's the part with the multiple glowing eyes and mouth. Always aim in the general direction of the head, even when it's not directly visible, as Armor Piercing may still get you a headshot.
Banshee - Obviously the head is a major weak point. But if you're not sure you can hit it, just aim for the stomach. It does no extra damage, but it's a large, easy target that doesn't move around much even when she flails about screaming.
Ravager - The sacks cause a burst of extra damage when popped, though of course this also releases Swarmers and spews their caustic crap in the immediate vicinity. Shooting for the head with an Armor Piercing weapon from a front-on angle may result in multiple hits.
Scion - Shoot the larger shoulder until it pops, then go for the head. You can hit the shoulder from the opposite side of the body, if you have Armor Piercing weapons.
Praetorian - Instinct tells you to shoot them between the eyes. Don't bother. Shoot the armor plates on the underside of the body, beneath their 'beak.' When the Barrier is down, enough damage break the plates, causing the Praetorian to fall to the ground and recoil, and revealing a bunch of Husk heads. Shoot the Husk heads and the Praetorian will die very quickly.
Combat Engineer - The head is obviously vulnerable. The turret they carry on their back can be destroyed, causing a damage spike and a small, staggering explosion. With Armor Piercing weapons, you can destroy the turret from a front-on angle even when it's not visible by shooting it through the chest area.
Cerberus Turret - As far as I know, it has no particularly weak points. But the thing beside the gun makes for a large, easy target. The base of the gun is the largest target that remains constant no matter where the turret is aiming.
Atlas - The Atlas has a major weak point on its back in the form of an exhaust port. It's bright and shiny; you can't miss it. When the Shields are down, the armor plating on the shoulders, knees and groin can be destroyed to cause a damage spike and force a stagger. Otherwise, just shoot at the front canopy. Hitting it with Armor Piercing weapons, especially towards the top or bottom of the canopy, may cause shots to hit the Atlas multiple times for extra damage.
Geth Pyro - Aside from the head, the Pyro has a cannister on its back that can be damaged when the Shields are down. When damaged, flame can be seen coming from the ruptures. The cannister will explode after a few seconds, killing the Pyro and damaging nearby enemies. With Armor Piercing weapons, this cannister can be hit straight through the Pyro's head or torso. If the weapon is sufficiently powerful (eg Widow, Javelin), a headshot from a front-on angle may be enough to drop the Pyro's Shields, pierce through the head, hit and rupture the cannister and effectively kill the Pyro in a single shot.
If you have more information on enemy weak points, please feel free to reply with details.
edit: Pyro and Engineer added. Prime removed. Was anyone ever really confused about the Prime anyway? Ravager headshot downgraded to yellow. Atlas canopy targets clarified. Exhaust Port location hopefully better illustrated.
Edited by OniGanon, 07 April 2013 - 05:21 PM.