I referenced the guide on TA for this and got it on my second try following their tips. I used the fight near the end of Chapter 8 in which you fight an Inquisitor with only Drax and Groot. For reference to the right point,
Do not stagger the enemy at all. Drax can perform his call to action without a stagger, Groot needs a stagger to occur. So if you do a stagger, you have a 50/50 chance of it happening, if you don't stagger then you just need the prompt to appear and you will get it with Drax.
I used a blaster from a bit of range and when I hit cooldown I would also fire the active reload shot. I did not use Drax's ability and only used Groot's if Drax was caught and needed to be freed.
My first attempt, I got the boss all the way to nothing and I was only given the triangle + circle option which would have killed the inquisitor. I gave up waiting for the prompt and restarted from last checkpoint to try again.
My second attempt, after I got rid of the first health bar and the extra enemies turned up, I got the triangle prompt and Drax ripped his arm off.
From all the guide reading I did, this one sounds like a nightmare, so combining these tips helped me get it pretty quickly.