There are two ways that come to mind that this can be done.
1) Create a separate, hidden counter.
2) Whenever damage is passed over, add that amount to the hidden counter instead of the lifebar.
3) Create an event like this:
This will slowly deduct from your lifebar that is visible whenever the hidden counter sees that it has damage added and allows you to use your existing counter image.
* Hidden Counter > 0
+ Restrict actions for 00''-05
Hidden Counter : Subtract 1 from Counter
Lifebar : Subtract 1 from Counter
Use the "Interface Bar Object" and tick "Smooth Bar" and adjust the "Count Speed" in the properties. This will allow you to customize how fast or slow the life bar moves whether counting upwards or downwards. You can use images also; one image for the background and one for the counter portion itself. It's pretty flexible.