That'll be it. I've just done this and if you hit the Shrieker with too much damage at the start like you're used to doing and send it to the platform before killing both Knights I think that will stop it completing. We had to switch up weapons and get someone with Arc to be the Knights killer. Riskrunner is good for this because the Thrall slashes will activate the continuous fire perk. Frankly, it was a nightmare. Took us 1 hour 20 mins to get it right!