Actually I already tested something similar to what I posted in my first post and it does indeed work:

/assist [noexists] focus
/cast Seduce

This will automatically assist my main if I don't have a target, and then cast seduce. Once the assist is done the first time, I can switch targets on my main and then hit the seduce button again and it will re-seduce my original target.

The issue here is that once the target is dead, it stops working, as I still have a target, but I can't cast seduce on it.

So once thing I'd like to try, or maybe you guys can confirm whether this already works, is:

/assist [noexists] focus
/assist [dead] focus
/cast Seduce

I know [nodead] checks to see if the target is alive, but will [dead] check to see if its dead? Or maybe something like /assist [!nodead] focus or something?