Basically it's exactly the same as battleshock in AoS - add d6 to the number of casualties and subtract your leadership. Then remove the number of models from the unit equal to the difference.
I've played my Orcs in AoS and battleshock does as much damage as the opponent. It's not uncommon to lose 5-10 models a turn from a large boyz unit. In fact, we plan on it - that's the whole point of running units of 30.
However my experience in AoS is large units of fragile infantry is a recipe to throwing away a lot of points. the roll is irrelevant if you've already lost more models then your LD value!
The only saving grace will be whatever they do with Mob Rule. Making units larger then 10 immune to morale, or if it started with more than 10 on a 4+ the model does not run away, something..anything. Otherwise horde Orks is a non-starter from the get-go.
