Many novice artists get too hung up on doing "exercises", because they're easy, straightforward, and comfortable (compared to doing the real thing). They waste a lot of time without improving all that much, because repeating exercises out of context can quickly become mindless, people tend to lose sight of the real purpose (making good drawings/paintings), and instead keep drawing boxes to "practice" perspective or get better at drawing straight lines.
I think what you need is a combination of what he's calling "scrimmage" and "drill".
To develop skills the fastest - try to do the thing that is as close to the specific real thing you want to do as possible. If you want to design characters - spend most of your time designing characters. You won't find a way to grow faster than by doing exactly what you want to get good at.
Then you can analyze your artwork, find the skills you're the weakest at, and deliberately practice them. But still, do it in the context of doing the real thing.