Whenever you are fighting against a boss, your problem usually is that you might not have enough coins to use the good items at the "shop" right before those bosses. Although if you have enough coins for the dragon, the coins are away no matter if you won or lost. So... here is the solution: Just get the coins, take the dragon and fight against the boss, now, if you know that you will not be able to do it (i.e. you have just 1 heart left) just turn off your GBA, when you turn it on again, you will see, you still got your coins and you can try it again and again...
Don't stop here... Check out our complete collection of Pokemon FireRed cheats