Enemy Acid Wall block

[Ranakastrasz]

I am trying to figure out how I might make Spitter Acid hit Walls rather than what is behind them. I believe that If i give them a collision size, it might impact objects like shotgun shells do. However, I want to be able to filter it in two ways. First, it needs to only hit Player objects, and the Player. Second, I want it to either only hit Walls a fraction of the time, or else only hit walls with enough health. The second is do-able innately with Piercing, like Tank projectiles have, but I have no idea how to specifically filter out other Spitters, trees, rocks, doodads, etc.