Here's some nitpicking just for the sake of it:
1 -- we're shown that there's a device that can use large amounts of mana, to produce life. That is, monsters.
2 -- the MC can understand the device's function, and uses the spawned monsters for "resources".
If that's the case, why not just spawn blocks of leather, scales, ores, anything? They have the technology to literally create life out of mana, why not just bypass the necessity to kill a living being, and just create resources directly? That's especially the case, since the MC is allegedly the "best magician to ever exist".