You're forgetting that people may actually watch this and ask 'Holy Radiance spamming is hard? SINCE WHEN?'
edit: we're friends with Zbiggi, don't shoot me mods!
They were slowed by the glyphed shadowflame.
Just look again 15:05 minutes.
However, we did not have time to kill the last bloods so we prayed for a miracle to kill the boss before berserk.
And we did it 10 seconds before berserk.