What you have withheld has nothing to do with it ... it is all about your federal tax bill based on your taxable income. If you want to use up the credit faster you need to increase your taxable income. Decreasing your withholding is immaterial for this issue.
Increase your income by earning more, sell off stocks at a profit, take out more from the IRA or retirement accounts and/or convert a traditional IRA to a ROTH. Sitting down with a local tax pro and/or a financial planner may be what you will need to do to get the best benefits in one tax year. However, if you don't use it all up the excess credit is carried forward automatically.
