Here are a couple vids off a machine.
The drill shown in the Stone video is a good start. You can then perhaps add in a "Happy Gilmore" drill as shown below
(trying to copy the movements as close as you can)
Your DD's swing needs to be more athletic for a lack of a better term. Any athletic movement, such as swinging a bat has a phase where the body is made ready to deliver the movement with as much power/quickness as possible.