I don´t really get why you´re so frustrated with it. If the grot player has the choice between 5d3 guaranteed mortal wounds over the course of a battle to larger units, or a relatively poor chance of taking out a hero, it probably wouldn´t be anything resembling good play to focus on the hero.
With the chance of hitting the hero being 1/6, with an average wound output of 2, it would take 18 battle rounds to reliably take out a 5 wound hero, and it would happen at the cost of causing those mortal wounds to other units.
It´s true that it could still happen, but it doesn´t really seem worth it to me.
If you´re still afraid that it would happen, as far as i know most factions have artifact options that either offers some sort of mortal wounds protection or healing, which would further mitigate the threat.