Better put it on a weapon. On a Kamihime the +1 only benefits that specific Kamihime, on a weapon your whole team benefits from it.
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I am elementless and SSR kamihime-less, and will eventually be replacing all SSR weapons of the wrong element with SR weapons with level 10+ assault. +1 on a weapon will affect only one team out of 6. Granted, that probably stacks with Assault and is the best choice.
Compare that to putting it all on Beelzebub. If she is in every party (and she will be for the foreseeable future) I boost 1/5 of the team always. Eventually, she will not benefit from the weapons in the team, and she will be the weakest on her own. She needs the permanent boost.
I would say Beelzebub is the only character I have worthy of every party. The next closest is Belphegor. I hope by the time I +99 Beelzebub, I can choose an element because I have a LOT of work ahead of me.
Unless you plan on using 0-Star SR-Weapons for all those grids, this will take a long time. Additionally i doubt that a grid with 10 assault weapons would do more overall damage than a grid with 9 assault and 1 +99 stat stick, which would make that weapon useful for all teams, at least until we get those accessories or your grid is full of max LB SSR-Weapons.
With a full element specific assault grid, every R-Rarity Kamihime would probably do way better than Beelzebub in that respective team, making her useless in 5 out of 6 teams. The +99 wont change anything with that.
I understand why you put it on Beelzebub and i'm not telling you to stop doing it, everyone should play the game the way they want to do it.
It's not as bad as I originally thought, but certainly it's slower than a dedicated element. I got 9 SR weapons (6 are fully LB, and 4 of those are assault) and one SSR weapon in this event. Being elementless, I will use all 9 weapons. I'm missing a few SR weapon types to use every soul in every element, but it's getting close.