Considering the max NP per day is like, 1,500np it's not a huge help to get it all, but every little bit helps, right? lol
Some, like Snowager on Easy, can SOMETIMES give you a Frozen Negg. These are rare but sell for around 270,000np right now. Snowy on easy has 700hp, but can do a bit of damage if you let him attack you. Trust me when I recommend against trying him on Medium. He has like, around 15,000hp on Medium, and can hit for over 3000hp damage. So you won't get your NP very quickly from fighting him on Easy, about 50np per battle. Most of the time it's junk for prizes. Usually frozen food. lol But, you can also get a couple of books and codestones from Snowy. I tend to battle him for the chance of a Frozen Negg or two.
I'll battle Snowy first, to try for the prizes, and then if I feel like battling after that, I'll take on something a little easier on Mighty difficulty like Jelly Chia or Shadow Usul. Oh, I don't know what the prices are anymore. But Jelly Chia has a couple exclusive items. Jelly Pop Stamp, Jelly World Stamp, and Jelly Mysteries, a book. I used to sell Jelly World Stamp for around 100k each, but that was years ago. lol The value is probably still much lower, but you'll get your NP pretty quickly fighting Jelly Chia on Mighty difficulty.