How can I track achievement progress?

Unfortunately, and unlike the achievement system in World of Warcraft, there does not seem to be a way to track achievements on the main GUI. You can, of course, continue to track it via the achievement UI, but that's not generally something you want to have open in the middle of a boss fight.

The thing about this particular achievement, though, is that it'll pop up as soon as you achieve it. So until it shows up and you get the toast, know that you've still got skeletal minions to let get killed.

If you click on the achievement from the menu, it will show you your progress.

You can tell which achievements can be expanded by the + icon.