i will qoute half the passage i got my codex bout 3 months ago on christams so
"Similiarly, weapons that get aditional Armour Penetration dice (such as chainfists, monsterouscreatures or melta weapons) do not get the xtra dice against the monolith. Ordanence weapons still roll 2d6 Armour Penetration and select the highest score.In practice, any weapon attacking the monlith will roll for Armour Penetration using its unaugmented strength and a single d6 no matter what.
also the powerfists double users strengt is augmented so you would only be using str4 or str 5 powerfists against the model however its users weapon so furious charge and toxin sacs would still be still fine as they add to the models profile unlike the powerfist wich uses double the models strength a.k.a augmented
"Similiarly, weapons that get aditional Armour Penetration dice (such as chainfists, monsterouscreatures or melta weapons) do not get the xtra dice against the monolith. Ordanence weapons still roll 2d6 Armour Penetration and select the highest score.In practice, any weapon attacking the monlith will roll for Armour Penetration using its unaugmented strength and a single d6 no matter what.
also the powerfists double users strengt is augmented so you would only be using str4 or str 5 powerfists against the model however its users weapon so furious charge and toxin sacs would still be still fine as they add to the models profile unlike the powerfist wich uses double the models strength a.k.a augmented