I don't run the game often but I am trying to nohit it (on brutal, I've already done it on violent) and have gotten to 6-1. With hideous masses I notice that sometimes I do like double the damage to them with no noticeable change in how I kill them (shooting the underside). I can't consistently replicate doing this damage, so how does it actually work? Is there a specific spot on it's underside that takes extra damage?

the red sections on the chest and tail of the him take triple damage