Setting a fixed custom difficulty using the password parameter. For example, if you want to set the difficulty to 128, you would use:

-o stratum+tcp://eu-merged-stratum.cminors-pool.com -u primary_wallet_address -p c=BTC,d=128

Here are the accepted values for the custom diff scrypt: 128 to 68719476736

Auto adjusting stratum's submit 5-15 shares per minute, if the difficulty is set higher it will be forced down to fit the lowest coin's difficulty.

( Fixed diff ports available by request )