Are use CorAfixx gel glue but it is cyanoacylate just like all the other gel reef glues. The trick is how you do it. Find a spot where your piece is going to be stable on its own or else build a little temporary support for it with other rocks or whatever and remember the orientation of your piece.Thatís really the hardest part because once you look away then look back itís hard to re-orient yourself with all those little jagged pieces of rock. Then put on lots of glue to the bottom of your piece and stick it on really fast then wiggle it around just a little bit to get the glue onto the live rock then just let it go and allow it to harden for a while. You have to get it on there really really fast because as soon as that glue hits the salt water it will begin to skin up and not want to stick to the live rock. Once you get it in place wiggling it around just a bit will break up that skin and allow The wet glue underneath to grab onto the live rock. I am new at this hobby my self and agree with you that it is very difficult to make those coral fragĎs stick as it does take a bit of practice. But I am getting better at it each time I do it. So the key is prepare the site and be sure the piece will stay stable on its own (you probably wonít be able to hold it still long enough with your hand), use lots of glue, get it on there as fast as you can, wiggle it a little bit then leave it alone for an hour or more. Good luck.
I have also used some stuff called Hold Fast by instant ocean. It is like epoxy putty. It works super great for pieces that are dry and out of the tank but I have not tried using it underwater. It says it Is Safe for Marine aquariumís but then it has all kinds of warnings to wash your hands after you use it so that makes me a little worried. I would get more feedback from people that have actually used that stuff in a running tank before I would use that underwater. Or maybe write to the manufacturer.