Suspiria has been shaping the world like a god, so as a writer it must be really difficult to not make something look like a Deus Ex Machina. I mean, the characters are currently dealing with someone who has the powers to create random Deus Ex Machinas all over the place as if they were in the Matrix and they need to figure out how to deal with someone like that. I think it's legit to assume that Crest's sword received an ultimate attack because of that.
While I do prefer Maytag and Bernadette over Crest, I also do prefer Mary and Suspiria over the others, so I'm perfectly happy with this arc. Okay, Mopey is still alive, but you can't have everything. At least Glyph died.
As someone who is also writing a story about someone very OP (the story is called Oracle of Tao, and there's a video game of it, and I'm trying to make a novel), there are certain tricks to it.
1. Extremely slow caster (Grune from Tales of Legendia, most summoners, Ambrosia in my game, etc).
2. Make them turn out to be a villain (Sephiroth, also Grune sorta, alot of characters actually)
3. Make them turn out to be a deity (also Grune, also Ambrosia)
And of course there's kill them off, make them the Ace (what TvTropes calls a heroic background character), or introduce Kryptonite into the setting.
But this isn't an accurate list because there are two others: make their powers have some sort of drawback or make them so strong that they have to hold back or blow everything up.
Ambrosia starts out with a sort of slow-cast tai chi power powered by tattoos, in the end of the first part she learns she is a deity but mainly what changes is that her power is now kinda normal speed and stronger, she can't use her deity powers (doing some means she takes up her power's responsibility and is hamstringed by its restrictions), and she get The Ace treatment until the ending where her daughter mostly takes the limelight.