In general I agree with
@Insahnity try to concentrate fire on one target at a time.
Depending on the mech, depends on what I target; Hunchbacks, Griffins and anything with an AC20, the right torso goes first (or the torso with the AC20 in it). Thunderbolts the CT goes first as usually I can get a critical hit on the ammo stored there. Rifleman/Jagermechs I go for the nearest arm/torso first as this usual reduces their effectiveness by about 50% same for Catapults, oh and Marauder left torso (take out the tac computer).
I tend to try to target the things I think are the biggest threat first, so anything with an AC20 or flamer, Hunchbacks, Rifleman, Jagermechs, SRM carriers, Cyclops (Z and HQ) and Marauder. Not encounter a Bullshark yet, but Annihilators are a nightmare, and even more so if more than one is fielded at once.
As for head shots I seem to find my Highlander-B is good about 50-75% of the time, but the star is my Marauder 3R, it can head shot about 80%+ of the time. The 3R is kitted out with a PPC (+10dmg) and AC10, a SPPC (++ I think) and medium lasers. I find if the PPC or AC takes the head down to the structure then the SPPC finishes the job almost 100% of the time, in fact it seems to work for legs too, though these have a bit more armour in general so its slightly less effective.
Your milage may vary as I am not great at this game, all my pilots are maxed out on each skill and I havent been keeping a tally of called shots against result, so the percentages are what I feel is happening and might not reflect what is actually happening.
That said I am very happy with my 3R that I take it on almost all 3+ skull missions