You can use Street Smash's practice mode to beat any of it's challenges.
Now how many minds did I just blow/hammers have I saved?
The five hits means passing 5 people, not actually hitting things in the game itself.
Depending on where you live, it could have been easy or hard. I sometimes carry my 3DS around just in case, and I've had plenty of street passes from just going to Starbucks or even a baseball game. However, I got all my passes for the challenge during the 3DS open tournament.
I'm not planning on using any of my hammers...especially the custom items one since it will allow me to keep track of when I get all of them. As for 50 hours, I've played a match as every character, then a match as every character in every costume, then a ditto free for all with every character twice...having each costume appear in at least one match. Coming up with different ways to waste time in Smash is how I'm slowly getting time for it. Would be nice if I knew of someone with the game locally though...
Edit: "Play StreetPass Smash with 5 different people" is what the challenge said, so practice wouldn't work.