How exactly do elemental damage modifiers work?

[trikibasey2]

I'm a pretty new player and I'm still wrapping my head around damage modifiers. I'm playing as a skeleton archer and I found a robe which says 'changes damage: 30% nature.' I also have another item which says 'changes damage: 10% lightning.' Does this mean that when I attack, my attack will do 30% nature and 10% lightning damage? Isn't this bad because if i'm up against a resistant to nature enemy, i'll do less overall damage? Can someone explain this to me as I went on the wiki and I still dont understand this. Thanks everyone!

[visage]

This is a word-for-word copy of a question posted to reddit a week ago. Was there there something in that discussion that left you confused?