A simple rotation of the sandbag (or kettlebell, if no sandbag is available) while descending into a reverse lunge will challenge your strength and core stability in the transverse plane.
When you add in the power of a swing, what you wind up with is a really cool explosive movement – the swing requires decelerating the implement at the bottom of the movement before you explode from the lead leg into hip and knee extension.
Starting with a weight you can curl 12-15 times, do 8-10 reps on steep side of preacher bench, match those reps on the angled side, then stand and rep out.