In the case of casting a shadow using a light I’d actually recommend using Classic 3D rather than the ray-traced engine. You have more control.
However, I think you’d be best off in this scenario scaling your actor layer down a lot (maybe to 2%) on the y-axis. To start duplicate the actor footage, move the anchor point on the duplicate to where the actor’s feet meet the floor, set the y-scale on this duplicate to 2%-ish (change as you see fit), then add a Generate > Fill effect set to black (or near black, or another color if you want to simulate some ambient light, a dark desaturated blue often works well for outdoor shots for example), add a Fast Blur and crank it up until you’re happy with the result (you could add 2 Fast Blurs, one for the Horizontal Blur and one for the Vertical Blur), finally set the shadow layer to the Multiply blend mode.
You’ll likely need to tweak some things to get this looking right, but it should get you most of the way there. If the talent moves toward or away from the camera then you might need to animate the transforms on the shadow layer. Also if the talent jumps very high you might want to keyframe an increase to the Fast Blur radius when they are in the air.
Also see what it looks like with the shadow layer set to a small negative scale (-2% maybe?). In the end you may want to use 2 shadow layers, one with a small positive and one with a small negative scale.