For minimum time, perform 5 rounds of the following while wearing a 30-pound weight vest:12 squats.12 back extensions.12 sit-ups.12 push-ups.12 get-ups.*12 pull-ups.
My first version of this workout featured 10 reps of each exercise, but I found that didn't fatigue the muscles sufficiently, so I cranked up the volume by 20%. It seems reasonably difficult now. What may not be obvious from the list of exercises is that this workout challenges the core muscles more than any others.
For the record, my latest time is 18 minutes, 32 seconds, just 16 seconds faster than my previous best.
* To perform a get-up, start from a standing position, lie down into a supine position. Then, get up into a standing position. This exercise is surprisingly tiring when performed with 30 pounds of steel strapped to your upper torso.