Blood magic seems to gain levels when e.g. the protective barrier takes damage, and it seems to be based on the amount of damage it takes. Greylings, for example, didn't level it at all, but one hit from a troll got me a level up.

I think the plains would be a decent place to level up blood magic - lox, fulings, and deathsquitoes all do enough damage and you only need one of them at a time, just keep recasting when it goes down and try not to die. ;)

For elemental magic, I find the freeze staff works really well against seekers and fulings, either of which can also help level up blood magic, but yeah, it's slow going. Currently at 20 elemental and 6 blood lol.