Drill:
Bring the front knee high and back in front of the rear knee (show the pitcher your butt) and hold that position as long as possible as you move towards toe touch.
At the same time practice tip and rip drill with the bat held horizontally just over the eyebrows in front of the forehead.
The old cue; "swing level" has been misinterpreted for a long time. Swing level does not pertain to the barrel path, but rather the hands once they get into the slot - and the "thought" through extension. In order to pull this off, there are the hand articulations that were mentioned earlier in this thread.