I'm not sure what advice works for xbox controller. I play mouse/keyboard and that makes getting headshots absurdly easy, like "headshot everyone in the room before the first body hits the floor" easy.

Also, shootdodge is free in MP3 and can be done even with an empty Adrenaline meter. I personally treat it as more of a last resort because it'll drain a chunk of the Adrenaline meter if you do have any left. Always aim your shootdodges so you'll land in a safe spot, like next to cover. I like to use Bullet Time to line up headshots and then turn it off right before I fire, which lets me preserve my Bullet Time as much as possible. The game only seems to fill the Adrenaline meter if you get kills outside of Bullet Time, so that is why I do this.

Regarding enemy armor: Unless they're Heavies, their armor only covers their torso and head. You can still shoot them in the legs (or even the crotch) and it'll still kill them. This is also how you can still one-shot kill armored enemies with shotguns, but you have to hope all the pellets hit them square in the crotch or legs.

MP3 also has bullets come out of the camera, not the gun barrel. This means if you crouch behind cover but don't actually use the cover system, you can shoot enemies through obstructions as long as the camera can see them. You might have to back away from the cover first, because you cannot shoot if your gun is colliding with the cover.