hmm.. top deck manipulation would help, but thats difficult in boros. The only thing comming into my mind is
Scroll Rack.
It would most likely "only" work for the first hit, but increases the chances alot, since you can put back humans you've drawn.
Shapeshifters would also be interesting, but as I think about them, they shouldn't count as non-human since "every creature type" includes human. So they doesn't seem to be the key for the human - non-human ratio.
/edit: stuff to thin out the library like
Land Tax would also increase the hit chance, but I think that's still not enough, even with 20+ basics that can be fetched.
Land Tax would also become a priority 1 target for removal (even compared to "common" games where you play it).