Basically you need 4 things.
One Damage variable
Two enemy sprite
Three text object/plugin
Four Keyboard/mouse plugin
Set the damage to whatever you want. Create a enemy with an instance variable for its health. Then set keyboard/mouse to subtract damage from the enemy's health. While spawning a text object with whatever coordinates you want (next to/on top of) for your sprite. Then setup the movement for the text object with every __ seconds moving the text object up -Y. Add a fade behavior to the text object. And your done.
Here is the example I put together to test. Floating Damage
Not 100% sure this is the best way, but it will get you started.
Here is a screen shot in case you can't open.
<img src="http://dl.dropbox.com/u/5773521/Fading_Damage.jpg" border="0">