In Double Dragon's Mission 2, right after you get all your hearts, you probably want to continue on with the game. Go a little further in, and you fight Chin.

Now, you could just beat on him until he flashes and disappears like normal, or you could go back down the ladders. Go down two ladders and he dies somehow... I don't think I'm up to rationalizing it.

Or you could use one of your new abilities to grab his hair and toss him off the side.

Either way and you'll be well on your way to the next Mission.