If it is UI, you can use two progress bars - let’s say progress bar A and progress bar B. A is going to show actual HP left, while B is going to aim for the value of A. This way you can tweak the speed of convergence and use different color to show damage dealt.
P.S. you can use “reply” or “add new comment” buttons to keep the question clear, so that anyone stuck with the same problem had no difficulties reading this in the future