If Gromril’s curse get’s activated on Ironbreaker that counts too. Even Handmaiden’s dash, Zealot’s dash and Slayer’s jump will give some amount of knockback (with HM dash having the lowest; not counting the obvious staggering movement ults of Footknight and Battle Wizard). To have this work out super safe you best get HM, Footknight with his block cost reduction aura and also having everyone taking additional 30% bcr. Just stand behind a pillar in a line with block up and wait for his phase to begin, and only start attacking him when he drops to his knees, when hopefully all adds have been killed. This takes a bit more time of course.
It took my team 3 tries too, first try one accidentely pushed one enemy, second time a fanatic walked into the backstabbing shade and eating the attack meant for Nurgloth, 3rd time worked like a charm.