You probably made a bad move by using a Golden Hammer on Diddy Kong's challenge. His challenge is all about being patient with KOing the rival, and there is a challenge where you have to KO your rival 4 times in a single Rival Smash. You could potentially kill two birds with one stone as Diddy, if you're patient enough.
Anyway, you're only given up to 5 Golden Hammers, and there are a couple challenges that can easily warrant their usage.
- Clear Solo Classic in 12 minutes at intensity 9.0 while playing as Marth.
- Clear All-Star on hard difficulty with all the characters.
- Clear Classic at intensity 9.0 without any customizations.
- Clear Classic at intensity 7.0 or higher with all the characters.
Of course, it's probably a better idea to hold onto your Golden Hammers until you're at a point where you can use them without having any regrets. Sadly, there's no point of using a Golden Hammer on the 8 KOs in a single Cruel Smash challenge, since you pretty much "have" to do it as Lucina anyway.
I probably did, but I got so frustrated by that challenge that I had no other choice. I KOd my rival 4 times with Mac though because Mac is a glass cannon and lends himself well to a mirror match vs the AI. As for the challenges you mentioned...
For my "clear classic on 9 without losing a stock" run, I attempted it as Marth. I got as far as Master Core without losing a stock, but then I died. Pressed on anyway, cleared Classic, and whoops cleared the challenge yay!
Clearing all star on Hard with everyone was too much for me. All star on Hard is daunting. I hammered that one. No regrets
Classic on 9 w/o customizations was tough, but I beat it on my second try with Bowser. Love that big lug.
And I had to clear classic with everyone anyway for the trophies, so not only did I beat it with everyone, (I used customs and broken equipment to make it easier tho no regrets) I did it with the majority on 7.5, allowing myself room for a mistake.
Also, when I used the Hammer on Diddy's challenge, I had 6 challenges left to complete. The other 5 are the 50,000 blocks left one, collect 700 trophies, 10 man smash in 25 seconds with everyone, get all the custom moves, and get all the custom hats. I have 2 hammers left. 700 trophies will be done since I got every randomized trophy, so enough Final Smash trophies will get my count up to 700. I also want to get all the custom moves, challenge or no. Im using a hammer on the Hats later, because I dont care about custom hats, and am saving the last one probably for 10 man smash in 25 seconds with everyone. I'm in a really good position, challenge-wise.