There is no way the dad is weaker than the average monster, so can’t they just first clear out the weaker ones immediately using some sort of magic? I mean if the dad is somehow currently weaker than your average monster, I would say he would probably be dead
Also,it doesn’t have to be a nuke. If Lloyd just wants to make and learn and research about magic, can’t he do something like creating magic that locates specific beings? He could use it to determine what is and isn’t a ally, and he has the ability to just individually attack each monster, it doesn’t have to be a big attack, it just has to be enough to kill the monster.
He has an angel as a pet, I’m sure he could clear out the whole horde, and call it literally what you said, God intervention, as he literally has an Angel to use. The Angel could just manifest or something, and just say some sort of speech on how he used his power to help them because of some bullshit reason