No missable achievements (plus 33 unknown)
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Escape the apartment block raid.
Get the crowbar.
Get the airboat.
Get the airboat's mounted gun.
Destroy the hunter-chopper in Half-Life 2.
Get the Gravity Gun in Black Mesa East.
Kill five barnacles with one barrel.
Use the antlions to kill 50 enemies.
Gain command of a squad of rebels in the uprising.
Survive the second turret standoff in Nova Prospekt.
Get through the toxic tunnel under City 17 in Half-Life 2.
Survive the generator plaza standoff in chapter Anticitizen One.
Destroy six gunships in Half-Life 2.
Shut down the supression device by disabling its generators.
Survive the rooftop strider battle in the ruins of City 17.
Find the HEV Suit Charger faceplate in Eli's scrapyard.
Kill 3 enemies using the crane.
Pin a soldier to the billboard in chapter Highway 17.
Disintegrate 15 soldiers by throwing them into a Combine ball field.
Break the mini-teleporter in Kleiner's lab.
Kill all of the snipers in City 17.
Discover the hidden singing vortigaunt cave in chapter Water Hazard.
Use DOG's ball to make a basket in Eli's scrapyard.
Kill an enemy with a toilet.
Kill five enemies with a Manhack.
Play through Ravenholm using only the Gravity Gun.
Cross the antlion beach in chapter Sandtraps without touching the sand.
Find all lambda caches in Half-Life 2.