This is actually not a particularly strong spot to use Radiant by default because bosses don't AA during tank busters. Then again, bosses don't auto during mechanics so there can be a lot of bad spots to do it.
I use Radiant on pull no matter what because it'd scale off FoF/Berserk and put it on CD. Otherwise used when it's off CD and it will live out most of its duration, otherwise better to keep it for the next phase/pull/whatever.