Shoot the monsters, magic to the heroes and heavy hitting units for the mortek. Or just go for the objective with everyhting you have. And remember, the harvester has to be within 3 inches to the unit to revive it. And about magic, fyreslayer have none, just one mortal wounds anti horde prayer, I´m pretty sure SCE have something similar.
Ballistas are too unreliable for my taste. I understand that you´re playing on table, so getting better units is gonna be expensive. You should try playing on tabletop simulator, that way you will learn which units work better. Also remember, mortek can´t reroll saves on shootings. Bonereapers are very slow, except for the cavalry. Once you control one victory point, it´s yours until your enemy gets it. So sitting around VP isn´t worth it. You should just try to hit one of your enemies VP with everuthing you have, after you´ve dealt with the cavalry. Don´t try to kill everything, the main objective is controlling the victory points.