About the only way to frag it would be with a chisel or sharpended screw driver. Very carefully prying healthy pieces away from the rock.
Before you do that though, are you sure the coral is bleaching (what signs?) and have you tried to figure out what's causing it? Even if you frag the coral successfully, there was a reason this happened. If you do not resolve that issue, the problem will continue.
Cheers
Steve