Well, since Slann isnt mounted (ie. cavalry model or what they are called in 8th), and it is classed as infantry in 8th rulebook, and there isnt any special rule that woul deny casting beasts 6th spell to slann, I dont see any problem Yeah, It sounds crazy but so does T10 Stank too.
All the book says about miscasts that happen without rolling double 6 is something to the effect of, "the spell is not cast with irresistable force". So I think that means your spell still goes off as normal if you meet the casting value. Fairly sure the miscast damage won't be reduced by magic resistance. It specifically refers to "damage from spells".