Setting a fixed custom difficulty using the password parameter. For example, if you want to set the difficulty to 64, you would use:
-o stratum+tcp://eu.merged.stratum.cminors-pool.com -u primary_wallet_address -p c=BTC,d=64
Here are the accepted values for the custom diff scrypt & sha256: 8 to infinity
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.