I noticed the Ratcatcher Triumph in my list as
22/25, but I don't recall what I was doing when I got that progress.
I have equipped both Europa set armour and also Season of the Hunt armour pieces, and tried killing yellow bar boss Fallen enemies in the open world, whilst rescuing the skiff, which spawns three bosses and got no progress.
I tried completing the Kridis Empire Hunt to see if a hunt boss would progress it, but that did not work either.
What activity do I need to do in order to progress this Triumph? It can clearly be progressed before the seasons starts on Tuesday 17 Nov 2020.