(above ) UHMWPE (AmSteel, Dyneema, et al), yep, amazing stuff, not just strong, but ultra low stretch, and better UV resistance than Kevlar. Excellent for making a Deltech cable on a folding frame.

But to the task at hand...

Wrenching on it with handle on one side only, has the drawback of wanting to bend the spine socket out of engagement, although bolting it in place should prevent that. But you are then still putting shear and bending moment into the threads, which will increase friction.

What you want is a "couple"; Either a huge 2-handed wrench, or as others have said, clamp the spline socket in a beefy vice* and grab the bike frame with both hands to unscrew it, and of course, making sure turning direction is correct. This will apply a "pure torque" with no increase in friction due to bending and shear loads.

* If the spline socket is clamped vertically in the vise, the vise may spin, unless the base has really strong clamp levers or does not rotate. If this is a problem, you may need to rotate the vise sideways and clamp the spline socket horizontal so is not trying to rotate the vise; The only drawback of that is that the vise clamps best at jaw center, not at the edge of the jaws.