Hi Reactant,
All you should be able to do is adjust the Damage Threshold to a value that you’d like. As an example 1000. Then set the flag for Accumulate Damage.
Next you’ll set your weapons applied damage when it hits the DM. If the damage is enough to exceed the damage threshold it will fracture and destroy the DM. If not, it will take a couple to do so. Let’s say your bomb only applies a damage of 500. It would take two bombs to destroy the wall with the damage threshold at 1000 and the accumulate damage set.
I hope this helps! 