Mawlocs vs. superheavies

Where larger tanks prowled, larger bioforms emerged to prey upon them. The Baneblades and Stormhammers of Dymetrin’s heavy tank brigades found their hulls compromised by the stabbing limbs of the giant Trygons and Mawlocs attracted by their earth-shaking rumble. Bladed scythes pierced armoured hides that their commanders had thought inviolable. Strangely, though, the Tyranids would retreat as soon as they had struck, either snaking off into the yellowing mists or burrowing under the surface once more.

Shield of Baal: Leviathan pg. 80