Progenitor - Radiation
You can easily 2-4 shot Eidolon parts without needing a Riven or other expensive mods.
If you're looking to 1 shot Eidolon parts or the Eidolons themselves, use VS. I've typically have been able to 1 shot parts at x2.25+ and Eidolons at x4.25+. (Individual results may vary based on squad and other variables.)
Why Primed Cry Rounds you ask?
More Sheet Damage = Bigger Numbers
If you have any questions, want to sell me Girl Scout Cookies, or anything of that nature, comment down below.