@Mayple The answer to the Mawkrusha vs Onion is intermingling your layers to form a net. Destructive Bulk has you pick a unit to deal Mortal Wounds to, so if you've threaded your units together it won't have a choice but to hit multiple models from different units with it's base but can only allocate Mortal Wounds to one unit, hence you've locked it up. I use this with my Arkanaut Company, so they can in turn shoot things off the board that get caught in my net (I call it a net, but I like Onion too)
This is also a great answer to denying your opponent the double turn of getting through your net and hitting the unit you don't want in combat.