In case any Witchbros don't remember, you should buy these crystals before reset if you can. If I remember correctly, these are the rarest UE enhancement material. I think the potion and the rune on that same line are also UE enhancement, but I think you can get a lot of those from several sources. I don't remember having an issue with those, but I do remember having an issue with the crystal things.

Also, you should maybe consider getting the Amanami UE first. I already posted about it, but her UE lets her deal damage based on 1% of the boss's max HP every second or something like that. I can deal 1.5m damage to my current Guild boss, and that's only like 1.5% of its HP, so I think the % damage will be very helpful for killing guild bosses in addition to being generally very good. For reference, the level 10 (max level) bosses for the weekly Evil Return Rebirth thing in global only had like 15 million HP.

Also potentially worth noting, Shuffle's UE makes her do the same thing that Amanami's UE does, except she deals 1% of the boss's *missing* HP. Depending on how retarded the guild boss's HP pools are and the time limit, an optimal setup might eventually be something like Amanami with UE doing the first 50% of the boss's HP basically, and Shuffle's UE doing the last 50% of the boss's HP. Unfortunately, Shuffle is an SSR doll not a UR doll, and I tested her in GL and she didn't seem that great, but also we were fighting bosses that "only" had 15 million HP, so maybe she just didn't have enough time to get truly set up for dealing massive damage, and she might actually be good on bosses that have 100 million HP.