There is only one method I've ever gotten to work 100% of the time for mounting soft corals. I start by making a "bag" out of some type of plastic mesh (gutterguard, plastic netting, plastic window screen, etc.) using either cable ties (zip ties), rubber bands, or fishing line. I put the coral inside the bag along with some small pieces of rock (a.k.a. "rubble rock"). Then I use another cable tie, a rubber band, or some fishing line to tie off the open end of the bag and keep everything inside. I let the whole thing sit in the tank until the coral attaches to a piece of rock, which usually doesn't take more than a few days.
Then you can simply remove the newly attached coral from the bag and mount that small piece of rock to a larger one, or just place it somewhere in your tank. Using this method you end up allowing the coral to attach on its own, and there's virtually no risk that it will tear away from the rock and end up floating around the tank.