"In its infancy, rock streamed through the planet in plumes of magma, spilled into hushed caverns, and quickened pools of protoplasm of its stilled inertia. Plates slipped and matter was crushed, pressurized. Later, as entropy bore cracks into the crust, sticky lifethings fused to the rock and scuttled forth. In geological terms, they are tiny things, but you insist that scale is the wrong one."
Tough enemy found in lower levels of Asphalt Mines.
While they seem to ignore lava completely, they don't like stepping in oil, so you can use oil pools to put some distance between them and you. You can also use the worker ants' acid clouds to put space between you and let you deal with the rather easier-to-kill ants.
Interestingly, great magma crabs can have bit of deviation in their attributes, which can cause them sometimes having up to 25 penetration value and lower or higher dodge value/mental armor. Their health always stays the same however, even if they have higher toughness.