Sniper rounds wound it on a 6.
@
OP
There's not much you can do but to just hit it with a lot of
AP 1/2 guns. If you have any insta-death weapons, that will be a good idea as Insta-death weapons nowadays causes 1D3 Wounds to a gargantuan. Just ally a unit of grey knights (i.e. paladins with hammers, warding stave and a Brotherhood banner) and you may kill it easily.