In Half-Life 2: Episode Two, there's an achievement for escorting a garden gnome to the end of the game and launching him into space. This sounds hard - and it is - but with a few tricks, it gets a lot easier. Also keep in mind the one extremely important point which you'll find at the end of the article.
There are a few places where you can safely leave your gnome behind, since you'll be coming back to those areas later. This really helps relieve some stress, since you won't have to take the gnome everywhere.
The first drop-off point is during the ant lion attack in the tunnels. Find a safe corner somewhere and stash your gnome for the duration of the battle. You can leave the gnome in this room for the entire trip into the tunnels with your Vortigaunt friend, so you won't have to worry about him at all for a huge chunk of gameplay. If you look at the image above, you'll notice that I set the gnome down right next to the table. That's actually a bad idea, since you'll have more Vortigaunts heading down there after the fight. Stash him in the back right corner of the pit.
Next up is the bridge where you get your car. When you arrive at the lookout point (with the rifle), look left and you can see the bridge. If you're quick, you can spot the G-Man walking to the left. Save here, and use your Gravity Gun to toss the gnome onto that section of the bridge. In the above screenshot, it's the leftmost visible portion of the bridge. Remember to save, because this might take you a few tries where you lose your gnome.
Minor warning: at the segment where you meet your first Advisor, your car is teleported to the gate after the "cutscene". If you left the gnome in the car, make sure to go back and find him where you left the car.
And now we come to the hard part: the hunter-chopper chase (also known as the worst part of the whole thing). There are a couple of potential strategies here.
First, you can just try to make it with the gnome in the car. I don't recommend this option because it's really hard. If you drive too fast or turn too hard, the gnome will fall out of the car and you'll have to stop to pick him up and reposition him. You'll take a lot of fire this way. And of course, if you drive too slow, you'll just get shot to pieces. Forget it.
A better option is to pick up the gnome with your gravity gun, fire him in the direction you're going, hop into the car, drive up to him, and then jump out and repeat. This is surprisingly effective if you're careful with your timing and only take chopper fire while you're in the car and moving. Be careful of the machine gun knocking the gnome out of your hands if you're carrying it outside the car.
The third option, and the one I prefer, is a bit of a physics trick. Jam the gnome into the back window of the car, like in the screenshot above. Accelerate carefully to a healthy speed, and then slam on the brakes. If you do it right, you'll get the gnome stuck in that back window, and it will be very hard to knock him out of there - you can pretty much drive normally at top speed. Keep an eye out because he could still fall, but you'll find this to be a very handy trick.
When you actually fight the chopper, be careful where you leave the gnome, because the mines can knock it all over the place. A good spot is under the train car you crawl through to get to the fight area.
After you beat the chopper you enter a garage. You can leave the gnome here while you disable the autogun.
And now you're just about done. Keep the gnome with you, and when you make it to the rocket, place him inside. But most importantly, remember to close the rocket door. If you don't close it, no achievement. I learned this the hard way when I finished the game again recently for my review, and I didn't earn the achievement because I didn't close the door. Worse, for some reason the game only keeps the six or so most recent saves, so I couldn't even load and close the door. I'll have to play the whole game again.
DON'T MAKE MY MISTAKE.