Player must wreck enemy gang members to get a 50x chain.
For these you'll need a 20x and 50x combo. Your combo multiplier will rise with each successful hit landed on any opponent. If you get hit in between, or spend too much time without hitting a enemy, it will drop down to zero and you'll have to start over.
On the first boss, you can knock him down and kick him to your heart's content with the Y button until the multiplier reaches 50x. It's easier said than done, because you have to press the directional button in his direction. If you don't do this, he will stand up and start to attack you.