Greetings,
this might be a lot of work for some but ...
In the black forest i subverted a greydwarf spawn not too far from a mined out copper node turned big hole and trenched a ever deeper path for them to enter then follow until they find themselves stuck in that large hole. while close it seems far enough away the spawner will keep producing like normal.
I can level my blood magic with the skeletons and shield. I also have a golem trapped in the mountains to train the other magic.
in both locations it is good to have a small rest/shelter structure to stay in for max skill gains.