I think the question boils down to "how much magic do I need to MAKE them plausible".
These are living things, so they need a metabolism to sustain their bodies. So these questions arise:
what do they eat to build up their bodies and regenerate wounds?
what do they eat, breathe or otherwise use for the energy to maintain their heat, fuel their thinking capacities and energize whatever their muscles are made off.
how do they actually move at all?
how do they procreate?
The first seems simple. They eat whatever rocks they require. The inside is hot enough to melt and the outside isnt heated so it cools off and solidifies, creating the outer "skin". Damage would naturally be healed by the holes that leak molten rock and hardens.
The second one is harder. With the temperature difference inside and outside of the molten rock monster they will bleed off a lot of heat away and would quickly solidify and die (assuming that too much deviance from 1200c will kill them). Maybe radioactive materials inside the rock can be excited for energy through the magic, but the careful balance of chemical reactions inside cells seems off-limits in a molten rock formation where there is little to control where and how the chemical reactions happen.
And their muscles? Pressure differences inside chrystalized formations of the rock? Pure magic?
Then the third option. How does it move? Its insides are molten so its outside needs to have the joints but extremely perfect joints to prevent leaking and any damage would mean both leakage and eventually the joint to lock up. Maybe some kind of chriystalline structure could worok but it wouldnt be very durable against damage. Again the creature would need copious amounts of magic just to function. And I havent even touched upon sediment forming and stuff floating to the top/sinking to the bottom.
And the last point, how would they ever get children? They cant carry them inside so they would have to build a mold and fill it with their own stuff and then... blow life into it?
It seems that for almost every stage of its life these creatures will require magic in some capacity. To move its arms and legs (indirectly through pressure or directly through manipulating their arms), they require magic to live, magic to move, magic to keep their temperature, magic to procreate... so they arent very plausible. That said, they are magic molten rock forms that somehow need something from humans enough that they try to hunt down and kill them, why would you need a plausible explanation for them to work at all?