For The Index. All Index enemies have alloy armor, which radiation, puncture, and cold are all effective against. You need a secondary that inflicts magnetic to deal with the shields, and ideally corrosive as well. Kompressa is an option.

RIVEN TIER LIST (after Electricity):

• Critical Chance
• Multishot, Damage, Critical Damage, Puncture Damage, Damage vs Corpus
• Fire Rate