At the end of mission 2, when the end-boss comes out of the door, climb down the ladder. After descending a second or third story, you should hear the sound of victory and you can move on to the third mission.
Play the first level of Double Dragon until you see the second set of double doors. Once there, take notice of the two vertical bars on the right hand wall. Kill all of the enemie, and then center Billy between the bars. From there, you can climb up and down without turning the characters back to you. If you go up, you can go off screen without losing a life. If you are up high, press left or right and you will "teleport" to ground level. Note: If you go too far down you will lose a life.
On the second stage, collect the whip from Linda before you climb the ladder. When climbing the second ladder, have the Linda knock down the player (the weapon is unequipped at the same time). Defeat the Lindas and pick up the weapon just before it disappears (pick it up on the second blink) and you will then have a bat to complete the stage.
It is possible to earn all of your hearts on the very first level. When you punch a Will, ynotice that he crouches down somewhat when you are hitting him. When he crouches down, that signals that the next punch will knock him down, so punch a Will until he crouches down, then let him stay up then go to the other Will and do the same thing. Keep taking turns punching them. Don't hit them so much at once or else they will fall down and die. Keep punching them until you have the desired amount of hearts.
On the second mission you can max out your statistics by climbing the fence, kill the Will at the top then drop down to the other side. Move forward and wait for Will to come close. When he reaches the end of the pipes in the background, climb back up the fence. Once done, move your character left until you walk to the edge of the yellow marker. Don't go too far left or the badguys will die and the trick won't work. (you'll now they died when the pointer tells you to go forward.) Then, move back and Will should be invisible. He won't be able to attack, but you can attack him many times. If you have the bat, simply hit the spot he was standing in over and over until you heart level is seven. You can keep hitting as many times as you want provided the time doesn't run out.
When you start the first mission, punch all the enemies you see. You earn hearts faster, for example, you usually learn to jump-kick at Missions two or three. You may earn the jump-kick by the end of Mission one if you punch people without stopping.