Try this
Go into Crucible Private Matches
Select Collision as the mode
Set the score to 25
Launch match
Cap the control point
Wait for the score to reach 25
Once the match is over, check to see if progress was made.
If so, then continue to use this setup to complete that part of the quest.
I just use the altars on the moon. Sometimes you can get a whole orb to yourself and kill all the enemies plus its even more likely now. I swear I did almost an entire catalyst (can't remember what it was for) while people were trying to get the witch off the wall. And I know it sounds arrogant but I went and knocked the witch so they could kill after I was done.
Since Whisper returned, I’ve defaulted back to that. Unless it’s specifically stating an activity or enemy type (a la Skyburner Oath), that’s one of the easier ways to complete catalysts. Endlessly spawning shadow thrall that all funnel into a narrow corridor at the very back of the room. Thrallhalla and the opening of Grasp of Avarice are also decent.
Then again, I don’t remember what was involved for Ex Diris’s catalyst, lol.