co-op-fail have the other person do every thing that is objective based and die 50 times.
Buddha: Play through and finish every level after Anomalous Materials with less than 5 points of health.
God: Finish every level with full health.
Be Ninja: Only use silent weapons throughout the entire game. (Crowbar and Crossbow)
Be Greater Ninja: Only use silent weapons throughout the game and do not take any damage from enemies (except Assassins).
Be Head Ninja: Only use silent weapons, do not take any damage from enemies (except Assassins), and kill all HECU Soldiers while they are not in an alert or combat state.
Be Master Ninja: Only use silent weapons, do not take any damage from non-Assassin enemies, kill all HECU Soldiers while they are not alerted, and finish each Assassin encounter within 1 minute.
Be Ultimate Ninja: Only use silent weapons, do not take any damage from non-Assassin enemies, kill all HECU Soldiers while they are not alerted, finish each Assassin encounter within 30 seconds and do not spend more than 5 minutes between each load point.
Rage: Break every breakable object in the game.
KABOOM: Successfully kill an enemy with every explosive that can be acquired in the game.
I’m Not Following YOU, Evil Person: Ensure the death of every Security Guard in the game.
Marathon: Sprint 26 miles.
Famine: Finish each level with less than 10% ammo for every weapon.
Energy-Efficient: Do not charge your suit, do not use your flashlight, do not fire the RPG with the laser guide active, and do not use the Tau Cannon, Gluon Gun, Laser Tripmine, or Satchel Charge.
Wild West: Using the .357 Magnum, score 6 consecutive kills without releasing the trigger.
You could run in circles 
Sure, but it would still take quite a while. 
you do know having achievements means some one is going to hack a server to where some one has to just type in some thing like !GAINALL and they will get every achievement. (3 servers like that in tf2. looking for an achievement server to get one achievement that was pissing me off.)
Ha, I can post you a pretty epic TF2 Achievement server when I get back home if you want. The admin bans a spawncamper every 5 seconds on average.
Camper - Stay still for 10 minutes.
Animal Rights Activist: Do not kill any Headcrabs, Baby Headcrabs, Houndeyes, Barnacles, Bullsquids, Ichthyosaurs, Snarks, Leeches or Cockroaches.
We’ve Got Noncombatants: Finish the “We’ve Got Hostiles” chapter without attacking any enemies.
Flying Swallow: Kill 50 enemies at point-blank range while long-jumping toward them.
Somalian Militia: Using the RPG, shoot down every Apache and Osprey in the game without using the Laser-guidance system and without missing.
CHOO CHOO: Run over 5 enemies with a tram.
Spam the R Button: Never fire more than 1 shot with any weapon before reloading.
Ultimate Achievement: Get every achievement possible in a single run without dying.
You Just Got RAMINATED: Find a way to get this achievement.
The Time is 8:47 a.m. - Ride the Train to Work
Don’t Let it Overcharge - Aquire the Gauss Gun
Prepare for Unforseen Consiquences - Survive the Black Mesa Incident
3, 2, 1, Blast Off - Successfully Kill the Tentical
Crab Meat - Kill Your First Headcrab
Him - Come Face to Face with G-man
Still think of some good ones…so more to come!
What’s the point of achievements when you’re just going to get them anyway?
That’s what I don’t like about the “Finish [[x]] Amount of the Game” achievements. No real effort needed. 
Well these are just like the easy ones to get, im still trying to think of some challenging ones.
If You Feel That You Have Been Exposed: Accumulate over 1,500 points of radiation damage during the course of the game.
4,000 Degrees Kelvin: Take over 4,000 points of heat damage during the course of the game.
You Don’t Dream in Cryo: Survive the Freezer with less than 5 points of health without sprinting or engaging any enemies.
Bones of Steel: Survive at least 50 damaging falls.
Mmm, Steak: Get a headcrab to launch itself at every hanging meat in Office Complex.
FEAR MY LASER FACE: Kill 5 enemies using the lasers in Questionable Ethics.
MY WORK=HELL die 666 total times in single player on easy.
xen class kill a vort so that its ragdoll falls into a chair
bullsh*t kill a headcrab then get killed by the bullsquid attacking it.
I am ninja get past the big tentacle things with out using nades
THIS IS BLACKMESA with a crow bar cruse the ragdolls of a scientist, Agrun and Hgrunt to fall of really big cliffs.
saved by the rent-a-cop have a guard kill something that is about to do a fatal attack to you.
Sorry if someone already suggested these, I didn’t have time to read every single post.
HAAAAAAAAX!!! - Survive a deadly fall using the buddha cheat
Overcooked - Kill yourself by cooking a grenade too long
Suicide Bomber - Kill you and one or more NPCs by cooking a grenade too long
Show Us What You’re Made Of - Put yourself in the anti mass spectrometer
Man, this is just like Counter-Strike! - Kill 100 enemies with a headshot from a pistol
Rocket-Jump - Fire the RPG straight down and jump 100 feet high
Does It Hurt When I Do This - Kill an enemy with a medkit
Lol you can’t gain achievements while sv_cheats is enabled.
For whatever dumb reason buddha doesn’t require cheats enabled in HL2, and neither does mat_fullbright.
Some more
Freakin’ Head Humper! - Die from a headcrab humping your head.
STFU - Listen to your suit for 30 minutes.
Friendly - Have an NPC follow you throughout most of the game.
Loner - Only have an NPC follow you when necessary.
My God, What Are You Doing? - Kill more friendly NPCs than hostile NPCs.
Emo - Suicide in every possible way.
I’m not sure if their A.I. is going to allow this, but I really hope so.
I think it’d be better if ‘friendly’ and ‘loner’ were more clever, like ‘social butterfly’ and ‘wallflower’, respectively. Or something. Also, I lol’d at the emo one.
Sounds OK.
I’m not sure how the system could tell whether or not you’ve been listening to the suit for half an hour.
Again, AI things can interfere.
Again… AI things.
I like this one, think it’s been suggested before, but the Achievement name is better.
That’d be quite a puzzle.