I'm posting this because I didn't find much posted about "I am not alone! IV" before I attempted it. For veterans this is likely old information, but as a newish player this would have helped a lot.
Description:
"I am not alone! IV" requires initiating 100 duels within the 24 hours between when missions reset.
Complications and mechanics:
You can get a free attack every 10 minutes.
However, this isn't often the case.
When your dueling energy is between 0 and 20, you gain 1 every 30 seconds.
After 20, you gain 1 every 3 minutes (or so, I haven't check to see if it gets even worse over time).
This means that to complete the success in the minimal time of 16 hours and 40+ minutes, you have to initiate a duel every 10 minutes almost to the second.
So, it's possible to do without diamonds, but ridiculously laborious. And I've already accidentally spent more than twice the reward amount trying this, and still might fail today because I realized these facts much too late.
Happy gaming!