Some ways to get credits:
- Advance Abbie's quest until you've met Emily. You can then get 20 credits from her once per day for cleaning her house.
- Sell items you win/receive as rewards at the item shop in town square.
- Play through chapter 2 and you'll get 50 credits from your first night at the strip club.
- After Chapter 3, enter wrestling matches at the Pussy Pit.
You can increase Missy's stats by playing the main questline. Currently you can get her to level 4 by completing Chapter 3. This will give you the best chance for a run at Poppy, but you'll still want to go in there with some decent healing items and probably some friendship bracelets for guaranteed breaks if you get grappled. You can further increase your stats with items like the friendship ring and lucky pendant. Some players have also reported that temporary stat boosts like mocha lattes and herbal stims have helped them.
Nonetheless, Poppy's still a hard fight so saving first is recommended if you're trying to defeat her (there is also an H-scene for losing to her.) It's worth mentioning as well that the first Poppy fight in the park can't be won or lost, as the guards intervene when either of you reaches half health. The real fight comes later at the summer house, so that's what you want to prepare for.