Assimilated Targets triumph, anyone know how to get this?

So checked triumphs for battlegrounds and saw this one, which is probably the typical one each new seasonal event that is annoying to get.

The description: Defeat Assimilated Demolishers in Echoes Battleground activities.

Well after 3 battleground looking for yellow bars to kill, I still have zero.

Anyone know what to look for to start getting these done. Braytech shows how many are needed, does not seem excessive unless it's the same as the one a while ago that does not show up all the time.