I believe you just need to replay levels when you get the opportunity to get more phozon and cash. Also a good idea for stocking up on potions you'll need.
If you're patient enough, you can create a Material that's at or above level 90 and then combine it with a Mandragora to generate 30 phozons. To increase the level of a Material, you'll combine two of them at a time, and the resulting level is a multiple of the two. For example, a level 2 and a level 5 will give you a level 10 Material. I think having the Spirit Orb equiped also helps here.
Also, don't neglect to eat food. The Health experience is separate from what you get from phozons and is the only way to increase your health. If you're late enough in the game, be sure to drink a Yogurt before consuming food or eating at one of the restaurants you'll unlock later.