Unless something changed recently, it should be possible to destroy the Dargyn without killing the pilot. Launching the pilot back into the air (in a ragdoll state) does count for the Riven challenge. (I think Stomp's lift is actually its own state/animation)
For this, I use a primary with no punch through (so that I can destroy the Dargyn by shooting at the front without killing the pilot), a low strength Nekros for a non-lethal Soul Punch, and Castanas/Talons as secondary.
Attach Castanas/Talons to the pilot, jump into the air and soul punch the pilot downwards into the ground, bouncing them off of the floor and into the air, then just slide and detonate the Castanas.
A little impractical, but hilarious (at least to me)