Source: Discord

Lilian Voss isn't counting towards Infinite Timereaver progress

There's a kill statistic for Darkmaster Gandling, but I'm not sure if that can be used in this case.

On second thought: Tracking for Ozumat uses the CRITERIA_COMPLETE event. If we can find the ID for the boss, it should be possible to add tracking for it in the same way.

All that's needed is killing the boss (in Timewalking difficulty) with the debug log enabled. It should print something like this:

Detected achievement criteria completion: criterionID