So I've been experimenting on how to increase the coins you get per fight, and I've found some pretty interesting things. This seems like as good a place as any to post this, though I might make a full post on Reddit or here later.
Spamming special moves only and getting 1 KO vs a lvl 1 random CPU gave me the lowest amount of coins, being about 9~. It actually had the coins individually rather than (Coin)x24.
Spamming tilts and KOing a lvl 1 once will net you about 20-30 coins.
This is where it gets interesting. Spamming back air only and getting 1 KO on a lvl 1 CPU nets you 50-60 EVERYTIME! I don't understand why.
I also tried a battle where I kept the CPU juggled in the air for as long as I could, but I got 23 coins.
Something with the back air makes you get a lot of coins, I wonder what this is tied to.
I played all matches with Mario.