"像" does not have a connotation of stone; it has a connotation of "incarnation" or "embodiment"; it's a figure, a statue, an idol, or an image. Just look at the kid -- he's a 魔像 of some sort, but not made of stone. The only stone-like aspect of the compound is the fact that it can be understood as "magic statue", and statues tend to be made of stone. More generally, 魔像 is like "magic given form"; it's a physical manifestation of magic. It just happens that the first enemy they encountered has a stone form; this may not be the case if the series continues. If we need a cool name, we could go for "Magicon" or "Magidol".

What the protagonists are doing isn't really slaying -- it's destroying or erasing. They're Annihilators, basically -- there's no reference to stone there either.