This has been so far the most annoying and RNG based trophy in the game. However, after spending a good hour or so on fighting the Inquisitor on Chapter 7 and the first one in Chapter 8 I found that fighting the Gold one at the end of Chapter 8 was the best way to do it since at that time its only Groot and Drax fighting alongside you. Unfortunately I didn't get a video of it but there is one guide out there that I will link at the bottom that when I noticed what happened I was able to replicate not too soon after.
Essentially it has nothing to do with staggering the enemy but a lot to do with Drax and the attacks he does. If you watch to the end of the video you can see that Drax baits the enemy into slam attacking him which starts a animation of Drax stabbing the enemy's hand into the ground and then holding the other one up, once this animation is happening you will then get the "TRIANGLE" prompt. (I'd also suggest not being too close to them both since it seems to be a dual attack between you and Drax in which you launch a range attack with the blaster while he pulls the arms off)
A lot of this seems to be patience, I personally would recommend maybe staggering the Inquisitor once at the end of his first health bar and then letting him stand back up. After that literally just wait and let Drax keep attacking him and eventually, hopefully he'll bait the attack and trigger the animation.
Hope this at least works for someone else because its by far the most annoying one, Good luck!
Link to video: