By shooting enemies. When I shoot them in the head, they died much faster - I think the first basic guy dies from 1 shot.
I had trouble killing the guys with minigun arms when not shooting them in the face - they took several shotgun shots in the chest.
Well, they normally take 1 SSG shot at point-blank.
In any case, even if you're right (because I can't just dismiss such possibility), there is very limited impact it has on gameplay, because most weapons are either projectile-based or very inaccurate hitscans, precluding precise aiming most of the time.
Are you a pro-FPS-player or some kind of a cyborg, maybe?
I think I'm about average FPS player, maybe slightly above average. Not by competitive DM player standards either. And I'm pretty certain it was the first time I launched Q2 in this century.
Counter question - ever tried playing any FPS game from comparable era on similar difficulty setting? For example for Q2 on hard try playing through Unreal, Half-Life, or Q1 on hard, or maybe Blood on Well Done. Notice the disparity in difficulty (manifesting in you getting face full of rape) between them and Q2.
Hell, my GF once put every Strogg (including a tank, gladiator, gunner, medic and so on) she could fit in cells behind the bars (well, forcefield) on jail level back when the game was new (without using cheats), just because she was bored. Granted, this loses a bit impact when you account for the fact my GF is one of the rare examples of female players who play FPSes (and is partial towards RPG elements, doesn't like TB, though) rather than Sims, and she can actually slightly outbadass me in DM (more than slightly in Q3 if we need to be this precise), but still.
Edit:
Checked. SG guard takes three hits from blasters on hard regardless whether you shoot him in the ankles or face, which is consisted with the information on wiki stating that he has 30HP and blaster does 10 damage in SP. No area damage multipliers detected.
Enforcer takes 3 shots from SG, similarly without any visible differences caused by hit area.
Also consistent with wiki information stating 100HP and max 48 damage per shell for SG, it's 60 per shell for SSG which is also consistent with enforcers biting the dust after just one salvo from this baby.
If I have save with RG anywhere near gunner may make a conclusive test - gunners have just 175HP, RG does 150 damage. If headshots (against aware target) do any extra damage, this should show it.