I've been thinking about what kind of baddies there could be that characters could blast away with impunity, so here's my pitch:
Chimeras
Chimeras are lab-grown monstrocities, byproducts of attempts to artificially manufacture top-of-the-line Linker Cores.
Extract genes from Linker Core-possessing specimens across the multiverse such as Dragons, splice 'em up, and grow 'em in the lab; focus on cultivating the Linker Core above all else, even at the cost of every other bodily function (not like they'll need them after extracting the Linker Cores). Some cooking in the nutrient liquid-filled tubes and out pops a Chimera: a reptillian-looking thing that shouldn't exist with more power than it can handle. And inside the Chimera is the real prize: a spiffy, top-notch Linker Core, ripe for grabs. Linker Core growing experiment success?
Unfortunately, mismatches in magic wavelengths from the multiple specimens used in splicing led to yet-to-be-solved developmental issues in their Linker Cores. The Linker Cores leak mana, and the maldevelopment of the Chimeras' body functions means that they rely on the Linker Core's mana to sustain their lives. Eventually, a Chimera will die on its own, breaking down into particulates all nice and neat. No corpse left behind, convenient! This method takes a while, since the Linker Core cultivation experiment was a success in a way, granting it a large reserve of mana to draw from. And though a Chimera lacks sapience with its tiny brain, it still has self-preservation instincts and will seek out mana-rich environments to delay the inevitable... Or, when it perceives the wavelengths of nearby Linker Cores: hunt.
Being a hybrid of some of the multiverse's most magical specimens is no joke. Chimeras possess significant, almost-but-not-quite-Draconic might, able to smash buildings with their arm(s) or tail(s) or whatever appendages that could be spliced on those things. And maybe, when desperate, a Chimera can unleash utterly devastating breath attacks at the cost of its remaining life. Their beastly physique makes them highly resistant to Belkan mass-based weaponry; but the lack of magical defenses leave them vulnerable to Midchildan spells. Dangerous and imposing, yet trounceable by magic children.
As for why Linker Core cultivation and the resulting Chimeras exist... Well, maybe they're from a facility in an ancient world under Belka's reign back when it existed, designed to bootleg some Linker Cores for Unison Devices? Or maybe Chimera-making was the whole point and they were used as bioweapons during the Saint King Unification War? Perhaps it's a more recent venture, where lab-grown Linker Cores could be used for Artifical Mages and they're rapidly approaching success in preventing leakage in Linker Cores?
I dunno, let me know what you think about this idea.