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.
Disintegrate 15 soldiers by throwing them into a Combine ball field.
Find all lambda caches in Half-Life 2.
Survive the rooftop strider battle in the ruins of City 17.
Destroy six gunships in Half-Life 2.
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.
Kill five enemies with a Manhack.
Kill an enemy with a toilet.
Use the Antlions to kill 50 enemies.
Cross the antlion beach in chapter Sandtraps without touching the sand.
Pin a soldier to the billboard in the chapter Highway 17.
Kill 3 enemies using the crane.
Defend Little Odessa from the Gunship attack.
Play through Ravenholm using only the Gravity Gun.
Escort Gregori safely through the church cemetery.
Use Dog's ball to make a basket in the scrapyard.
Find the HEV Suit Charger faceplate in Eli's scrapyard.
Get the Gravity Gun in Black Mesa East.
Discover the hidden singing vortigaunt cave in chapter Water Hazard.
Destroy the Hunter-Chopper.
Get the airboat's mounted gun.
Get the airboat.
Kill five barnacles with one explosive barrel.
Get the crowbar.
Break the mini-teleporter in Kleiner's Lab.
Escape the apartment block raid.
Put the can in the trash.
Hit the trashcan cop with the can.