I'm only playing on normal, but I'm like 85% sure I've seen AI squad members kill enemies on that difficulty.
TC you really need to use cover more. The AI is more accurate than you, so crouch (it's the B button) and hide behind walls, popping out to shoot them, or try to situate yourself so that you can aim at them, even just an arm or leg, but they can't see you well. Sure there's no button to make yourself stick to a piece of cover like it's glue, but work on it and you should start doing better.
Xbox Live: ElevenOHNOEZ .....................................PSN: nightranma 3DS: 5455-9525-0021
Shoot aliens in a corridor, shoot human enemies in a corridor. It's really not that hard even on ultimate badass due to the constant health, armour and ammo powerups unrealistically placed on every level.
Also sloppy ai programming means that all the enemies focus their fire upon you, it's that sort of game. The type of thing we see in lesser quality titles.
Idk what your talking about as far as "lesser quality titles", I've seen this exact same problem in pretty much every CoD game I've ever played. Only franchise I can think of that doesn't have this problem, at least not as bad anyway, is Halo.
And what I found effective in human sections if your having trouble, is to take your time and snipe with the battle rifle. Even levels that are supposed to be tense and fast paced (such as releasing the hangar controls at the end of thhe sulaco portion) have a limit to the enemy spawns, so you can snipe them all from a distance, then advance until the story triggers the next wave, rinse and repeat.
Aliens can be deadly at times, particularly on higher difficulties. If you find yourself having trouble with them at any point just memorize their spawn points and blast away.
You die on normal? I die on UBA with aliens which take tons of damage coming from all angles or enemies chucking a grenade down my throat or surprising me around a corner with a shotgun. Other than that, the game's pretty easy. Oh, and you can take cover.