The way I would do it is break up the animation into separate animation clips and name them accordingly. I'd set their wrap mode to clamp forever as well.

Then you make sure your model's game object has an animation component attached to it and it has reference to the aforementioned animation clips.

From there it is simply a matter of implementing a conditional statement in your script where you accumulate your damage.

Note: You might have to adapt this to suit your Game Object structure, as I am operating under the assumption that the script in which you accumulate damage is a component of the same Game Object that your animation component is attached to.

if(health < 20)
{
    animation.Play("animationnamehere");
}
else if(health < 40)
{
    //so on and so forth
}
else if(health < 60)
{
    //so on and so forth
}

Notice in my script I am working backwards, this is because if you did it the other way( if(health < 90) ) it will return true on that case for everything less than 90 and you wont see the state change because your logic is flawed.

Just a note that there are FAR more efficient ways to do this but I am working here for the sake of simplicity.

Hope that helps.

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