I just call them “The Marines” or “The Grunts” seeing as the “HECU” was never mentioned in HL and therefore is not canon.
Okay, I fixed it, damn, I sure walked into a shitpie calling them HECU huh?
Static target, moving shooter Headshot an Hgrunt while riding the tram in ON A Rail.
Man about town - Speak to every friendly NPC in Black Mesa
Bite this, sucker - Kill every leech in Black Mesa
:what:
No, they’re HECU. Does no-one remember that Opposing Force is canon?
oh u
Around 11 :3
bow-chicka-wah-wah
I’ve been lurkin’ around for some time now, and finally registered to the forum. (Obviously just to post on this meaningless thread, right?) So… hello everyone. 
Teamplayer: Don’t let any friendly NPC die IF you can save them.
Takin’ the hard way: Complete Black Mesa with Hard
Tongue Fetish: Kill 20(?) barnacles with the crowbar
Workout Day: Complete On A Rail on foot
Courage: Kill an ichtyosaur with the crowbar
Military Engineer: Take down 3 enemies with one satchel
How exactly do you think doing that?
I believe you need the train to breach the first wall (If I remember it correctly, long time ago).
So by breaching that wall would, you would already fail the achievement because you used the train.
My Ideas:
The only easy day was yesterday… Cause a resonance cascade. (Story line).
Demoman Destroy a tank.
Hard Contact Kill 10 Vortigaunts by using only a crowbar.
The first steps to victory Complete the Training chamber.
The sleeping lion Kill a headcrab without him ever known you where there.
Fire in the hole! Destroy the giant arm (Blast Pit) Without distracting it by using grenades.
that “Giant Arm” you mention in Fire in the Hole! is a tentacle (common knowledge, Noob)
:retard:
On A Rail starts after you breach that barrier.
Obsessive compulsive disorder Replace all crowbarred duct covers with new ones after you leave.
oh wait you can’t do that can you?
ALso more achievement ideas, less bickering.
That’s actually hilarious. It should only be one duct to get the achievement, because it would be impossible to remember to do that for every duct, but imagine coming across that.
Reminds me of some of the instances in HL2 where a wooden plank or similar would be holding a door shut. I always replaced it when I had passed through. Sometimes painstakingly so.
I leave them open in the vain hope that some rebels will come through to support me.
They never do 
Actually they may… I mean… I always blocked the doors. Oh God. What have I done?
I’d hit it with a crowbar- kill every xen monster that you can with a crowbar
zombie freeman- die by a headcrab on easy
it just got nuked- over kill a headcrab with an rpg
retarded military- bring 15 solders to an environmental death
god has a crowbar- kill a tank with a crowbar (its possible on easy)
stop the grenades please- have died 20 times by explosives
vortal ownage- kill every killable vort in the game
Boredom strikes again:
MO: RPG: Score 1,000 kills with the RPG.
Tau Me, Tau You: Kill an enemy player by ‘batting’ them back and forth with another player, both of you armed with a Tau Cannon, at least three times.
Gravity Pun: Kill an enemy by forcing them off a ledge that is impossible to survive.
Slaughter Underwater: Kill 5 or more enemy players with a Satchel Charge whilst they are submerged underwater.
Set 'em Up, Knock 'em Down: Blast an enemy upwards with an explosive weapon, before killing them in mid-air with the Crossbow.
T.H.H.O.A: Cook the Hand Grenade for three seconds before killing three enemy players with it. Five seconds is right out.
Lucky Punk: Get hit with a .357 with all six rounds without dying.
Science Affair: Win the round using only the experimental weaponry (Tau Cannon, Gluon Gun).