Dark Willow (https://villains.fandom.com/wiki/Dark_Willow_Rosenberg)’s powers can drain out but she could absorb powers from anywhere, so her weakness is also a strength as she can absorb (https://www.geekbomb.net/fight-of-fandoms-glory-vs-evil-willow/) Glory’s power.

Glory (https://buffy.fandom.com/wiki/Glorificus) has to absorb life forces from human beings or else she’ll start going crazy/weaker and probably isn’t able to win.