get something like chicken wire (preferably something plastic). make the hill then cover with mesh, secure the ends of the mesh with something heavy like a rock so it cant move then sprinkle more stone overtop to hide the mesh. iv never tried that but i think it should work, because thats how you reinforce banks to stop them falling apart.
Also, get an object (like a squashed circle sorta shape), cover with a thin layer of silicon and quickly sprinkle DRY stones over untill theres no more silicon showing, then wait for it to set and then place back in the tank and then cover with more stones. could also make an awesome cave that way