Quick trick. It might or might not work. But it worked for me most of the time.
Dig a hole in the sand so that the lowest possible point that the fish can go is the sand. Then slowly empty out the water from the tank into a large trash can bin. If the fish is "smart", it will swim to the hole. Then you drain more water so there's nowhere else to go and net him out.
Couple things to be careful about though.
1. Make sure there are no fish or inverts stuck between the rocks and exposed to the air.
2. While you drain the water slowly, also make the entire process as quick as possible so your liverock stays fresh.
Hope that helps. Even if the fish doesn't go for the hole, it will at least be much easier to catch with less depth of water to deal with.