Yeah, I saw some dudes who did that on YT. But I could not reproduce it.
I brought my A-game to that fight and even after pumping all of my rockets and 'nades into him, I could not get him down. I did go and grind out my missing perks then (the one that gives more rocket ammo). And I intend to reattempt this thing. Just need to conserve all of my rockets for that phase.
I used the LKW for the first stage so had all the rockets I needed for the second stage. There are also refills when fighting the second stage too if needed