Last I heard (which was years ago) elemental absorption of different values are supposed to stack. ie: 5% + 15% + 30% + 37% + 50% all stack (multiplicative, not additive, so 50% reduction, then the 37% is applied to the remaining damage that got through, then 30% of what's left from that, etc. or in other words: if same values stacked 50% + 50% would result in 75% total reduction, not 100%).
However I have not tested this in a very long time.
Also, yeah, spoilers. I did not know the name of the raid until I opened this thread, and I opened it because I did not know what forgewraiths were and thought it might be a typo or something (sometimes people use different names for things than what they actually are).