So, I'm not sure how applicable this would be in a speedrun, but I've discovered something weird about the game. Only certain objects are reset when you restart a challenge.
Fighting Fit is my example, because I hated that Jiggy casually, but I found a way to make it as easy as just... Sitting there waiting for Mr. Fit to finish his jog in peace.
What I did was just figure out where each Gruntbot spawned - There's a set pattern of them that spawn in specific locations throughout the challenge when Mr. Fit reaches certain positions - Placed a box where I thought it would block the gruntbot, and then spent a few replays nudging each box around until it was just right. This took a lot of trial and error, but at the end I was able to restart the challenge and literally join Mr. Fit on a casual jog up the hill, free of any concern of either of us being attacked. 1000 points, easy.
In theory, you could do this with just one challenge restart - Use the first "attempt" to place all the crates or whatever that you need, then restart and wait for Mr. Fit to enjoy his jog and hand over the jiggy. In practice, that sounds like a nightmare.
I found the execution to be very satisfying, if it took a little while to do, and I recommend trying this out for fun - Especially with Fighting Fit. Screw that challenge. I haven't actually... Finished watching a complete spedrun of the game, but it may be the case that this is useful in certain useful circumstances, too?