[JoeGuardsman]

You have that Carnifex dead on turn two with less than half your army. I don't know what you are worried about. If you need to worry about other things in the army you just shoot at it with the rail gun on the hammer head for three turns and when it comes into range bring up the pirahnas and finish him off bring in the crisis suits with plasma rifles for insurance if you want.
Unless Tyranid players get lucky with shooting uber-fexes are useless. Though they can be useful if your opponent doesn't have the hard ware to handle him.