Okay i keep telling my mechs to headshot with numberpad 8 key and all they do is sit there when i press it and click on enemy mech. Am i doing something wrong here?

Have you tried holding it down throughout the attack. I don't think it's a toggle switch (on/off). At least that's what I recall from play the game.

Quidquid Latine dictum sit, profundum viditur.
(Whatever is said in Latin appears profound)

its only numpad8 and leftclick but it is also a fire from current position command. the mechs will not move to engage the target and thus will not fire at the target if it is out of range and that is most likely your problem, try outfitting your mechs with longer range weaponry. a good example of this can be found on the first mission on the original campaign where you face an ulller w if you leave your mechs stock the uller will move out to long range with its ultra ac where you cant touch it and pound on the center torso of the firestarter until it runs out of ammo then moving to medium range with its lasers. at that point your firestarter will be able to engage it with its ppc.