Here is the full list of all 33 Half-Life 2: Update achievements. It takes around 8-10 hours to unlock all of the achievements on Windows.
Destroy the Citadel's reactore core.
Survive the rooftop strider battle in the ruins of City 17.
Shut down the supression device by disabling its generators.
Kill all of the snipers in City 17.
Survive the generator plaza standoff in chapter Anticitizen One.
Get through the toxic tunnel under City 17 in Half-Life 2.
Gain command of a squad of rebels in the uprising.
Survive the second turret standoff in Nova Prospekt.
Destroy six gunships in Half-Life 2.
Use the Antlions to kill 50 enemies.
Pin a soldier to the billboard in the chapter Highway 17.
Defend Little Odessa from the Gunship attack.
Escort Gregori safely through the church cemetery.
Find the HEV Suit Charger faceplate in Eli's scrapyard.
Get the Gravity Gun in Black Mesa East.
Destroy the Hunter-Chopper.
Get the airboat's mounted gun.
Get the airboat.
Kill five barnacles with one explosive barrel.
Get the crowbar.
Escape the apartment block raid.
Hit the trashcan cop with the can.
Cross the antlion beach in chapter Sandtraps without touching the sand.
Play through Ravenholm using only the Gravity Gun.
Break the mini-teleporter in Kleiner's Lab.
Disintegrate 15 soldiers by throwing them into a Combine ball field.
Find all lambda caches in Half-Life 2.
Discover the hidden singing vortigaunt cave in chapter Water Hazard.
Kill 3 enemies using the crane.
Kill an enemy with a toilet.
Use Dog's ball to make a basket in the scrapyard.
Put the can in the trash.
Kill five enemies with a Manhack.