Wiki says you learn Radiance from the exorcist after that quest, and I can verify that. So it doesn't seem possible to have Radiance for that fight. Either way, you can just Star Knightess the fight, assuming you got rid of the barrier first.
I think I have? Does it allow you to use Purify if you've dissabled the barrier? If so I don't have Purify and don't know where to get it... (Maybe it's just straight forward and it just disables it completely and I've just missed one of the things you need to destroy to get rid of the barrier despite having comed through almost the enitre domain if not all of it...)
The newb trap with the barrier is that one of the orbs you need requires a trick to reach: give the village alchemist 2 Pyromantium so that she has 10 bombs stocked (don't buy them). The next day Aaron will be at the quarry instead of in bed, so you can Open Domain on him and get to the last orb. Then Belphegor will be just be a single fight that you can SK.
The villagers' dialogue hints at it (iirc they say if there had been enough bombs Aaron wouldn't have been injured). Also you can see the last orb in the demon domain, you can't reach it but you can tell it has to be done from the quarry. There was supposed to be much more of this kind of puzzle here but it got cut, leaving only this one that almost everyone misses.