As I was walking into the Antlion room, I noticed that jumping down was the only way in, and there was no visible way back. "Hm," said I, "That seems mighty suspicious. I bet that as soon as I jump down, the whole place will flood with Combine and I'll get killed ten different ways at once. Let's make an escape route."
So, I whipped out my gravity gun and sucked over the big crates. Then I hopped onto one (and since I wasn't touching the floor, the Antlion didn't come out) to get a tiny bit closer to the barrels, and then I brought those over too. After a few minutes of work, I made a staircase out of them that let me hop back up to the entrance. Then I cautiously stepped out onto the floor, and the Antlion came at me. So I fled up my staircase, taking a couple of hits on the way, but managed to jump over the railing to safety! Then I threw submachine gun bullets, grenades, and rockets at the Antlion until it fell down.