Soft coral is tricky, it is so slimy that when coupled with a wet environment inside a tank it doesn't really give the glue anything to bond strong to. One thing that I have tried with some success is take a piece of dry rubble rock and attach the soft coral to that. Let it set, and then glue the rock to the rock in the tank. You may still have to get crafty with the way you glue the coral to the rock, I'm a big fan of laying a piece of my nephthea leather coral that I frag often horizontally in a crevice and then gluing a completely around the coral in the middle. Eventually the two ends will stand up and you will have two major growth points for the frag.
Some folks have also used pieces of zip tie skewered through the base of the coral and then glued the two ends sticking through the coral to the rock. Eventually the coral will grow over the zip-tie piece entirely. However, I'm not a huge fan of that method since it will look unsightly for some time.