I use a rock tumbler. I fill it about 2/3 full of coins, then fill with tap water. Usually 6 to10 hours of tumbling will do it.
Seperate the copper coins from the others or the clad coins and nickels will turn orange. Don't attempt to clean collectible coins!!! Use this method only on spendable coins.
Good luck.
You could try freezing it in distilled water and then break it out of the ice. Repeat until as clean as the ice can make it. Some museums do this to clean specimens.