Took me 3 weeks of all sorts of different methods to catch a cherub angel in a 180. BUT, when I finally caught it, it only took 15 minutes using the "clear bag" trick. TRUST ME!
Large sized freezer or plastic bag, clear! Purse string some fishing line around the outside of the opening of the bag. Place a rock in the bag, or wedge it into some LR. Put some of the angel's favorite food into the bag. Shut down or reaim any powerheads that will blow the food around. Run the fishing line out of the tank and into your hand. Sit low and wait. Be prepared to see every fish you are NOT trying to catch, swim in and out of the bag. BE PATIENT! The fish you are trying to catch WILL swim in there eventually.
Once he does, yank the string as fast as you can. It will sinch the bag closed with the fish inside. I've used it several times since learning it. It is a method that can serve many a reefkeeper well.
Good Luck!