Hi,
With xenia, most of the time you can just put some rubble rocks around the colony and they'll spread over and then you can cut/separate them without having to do anything.
If you need to cut them, I've found the best is using a loose netting like bridal vail and loosely tie them down to a frag rock. If you put too much pressure on them they usually will try and push off or through.
Hope this helps.
thanks