SONIC UNLEASHED

To get the required rings i took the following steps:

1. Find Wentos and 99 buy chocolate bars for 50 and sell them back to him for 100 and repeat untill desired.
2. Go to Mazuri and buy 99 strawberry cake for 60 (30 exp/each) and 99 chili dog for 50 (30 exp/each).
3. Repeat the process untill you have the required exp.

Note: Wentos does not always have chocolate bars on him and he does move after you talk/purchase from him. He also does not always give you a profit for chocolate bars, sometimes he will try to buy them for less than what you pay for them.

Simply earn enough EXP to boost your Werehog's combat level to 20.

EXP can be earnt as Werehog Sonic by defeating enemies and collecting the yellow diamonds that they leave behind. Or as Sonic for completing levels. You can then use the EXP to boost stats at the end of a level.