I further elaborate on my post located here: Sean's Epiphany - Wait. How am I going to do this?
First, I think you did a great job. Better than I would have.
And of course, having said that, here come my comments.
As far as tracking which monster has how many HPs, if it’s a flurried and hurried combat, I do tend to let them be random, unless someone makes a point of focusing fire. Both ways, from the monster attacks and the party ones. In my experience, it keeps people from getting TOO bogged down, just swing and go. Also, if you feel the combat is going one way or the other, you can spread damage, or focus it instead of random, to help the flow of the game. I’m not going to fudge die rolls, but I think this can help from one side being cleanly wiped.
But I think over all you straight up nailed it. That encounter was built to suck HPs. And it did.