Umm... You don't know what I'm talking about
Um... if you make an object plain, then texture it with whatever your going to shoot, you could move them around with the position object, delete them if they get shot, etc.
Or... I think you could do it with sprites. I think that that's what your after, in which case my knowledge is very limited. Actually, I'm not certain as to what you mean exactly. Do you mean like Contra, (Think NES Mario with a gun) or do you mean like Doom, (Monsters are 2d, but the environment is 3d) probably the first right, or closer to it?
Destroy you with my mind, I could!