Before looking the rest of the contents be advised as a tradition keep Gnome Chompski with you. He is waiting you at the very start of the level.

Campaign related. Can't be missed.





Can't be missed? I mean first chapter you just open the sealed gate by shutting down the electrical socket with bare hands.

This simply requires you to get G-Man's sightings. If you are unlucky with these, then no fret. First one is inside sewer area here:







Once you get outside and meet Antlions coming out from the sandy grounds, make your way through the building with Rebel Resistance greeting you at the gate. Once one of the Rebels opens the gate, it will open another one for the mountain entry on the right. Follow trail up and immediately rush it to view on the left where G-Man is leaving. Where APC Vehicle is parked.







This should be after the first G-Man sighting. I'm adding this here because... some fat chances are you'll kill this zombie before you let it burn. Screenshot below if anything.





This is located in the area where you'll have to use the Gravity Gun to open the locked door. Beforehand it's basically where you obtain the Magnum. As the title of the achievement hints: break it.






This is simple as it can get, you just solve the combination hinted below on the piece of paper so you can unseal the gates blocking to the tower. Make sure to save before doing this cause you need to do it in one go.



Oh come on, even a toddler can solve math easier than this. Do it on your own.



This is located below the coast shore of the Lighthouse (Point 55). Behind the house. Just climb the slide and slide down. It should be yours.






While at the Coast, when you arrive the entry of the Lighthouse... you can immediately see a random figure appearing on the top of it.







Inside the mysterious (and spooky) Residence you'll find lots of car batteries, just spare one and go with it outside the small shack where locked cage is containing the key. Simply grab the key and the rest of the batteries and use them at your free will on 2nd floor of the house.







While inside the mysterious Residence area, on 2nd floor in the back you should be around this area and will come across this white door (the bathroom) open it and well, do your dirty work.







If you've made it to the very end with Mr. Chompski then Congratulations! Throw the poor bastard into the core once you've activated the override sequence as everything blows to hell. Suggestively make a save before doing this at the finale and once the window gets broken by the core laser just yank it away with the Gravity Gun.

These are very easy achievements to get but some can be missed either way if not attentive. In any case I hope this helps! Thanks for reading.