I think that the best way to deal with this Raid Event is to farm Assist Raid with BP rather than abusing Half-Elixirs on Standard, which from what I can tell doesn't give much Divine Soul compared to Expert (got 10 DS out of 10 Expert Assist/ got only 3 DS out of 10 Standard Solo). Furthermore you get a fair replenishment of BP from the Event Gacha so you can go all-out with it. Concerning Ragnarok and Devil Soul, it was wayyy easier than I thought it would be, already got about 900k PP and 21 Devil Soul without spending any BP aside from Burst Time. I'll surely be done with it in about 3-4 days. I guess that the only thing that will eat up most of my BP is that 200 Divine Soul.