Q: "Why use [Amalgam Argonak Metal Auger] at all? Why not just strip armor with abilities?"

A: Some units CAN'T be stripped by abilities but CAN be stripped by that mod, which frees up a slot on your dagger melee since you don't have to use Shattering Impact -- if you use that anyway, they both stack, resulting in even faster stripping -- also the +3 Punch-Through & unique effect of HIGHLIGHTING ENEMIES BEHIND WALLS WHEN AIMING really helps in lots of tilesets

Q: "What do I use if I don't have that riven?

I recommend [Thermite Rounds] and [Primed Cryo Rounds] (or [Rime Rounds]) in place of the riven and [Rifle Elementalist]; you will want a companion with [Tenacious Bond] in this scenario (and [Reinforced Bond] in any event)