Aw, geez ... that's not easy. She doesn't have "a" power, per se -- she's a sorceress. So she basically has a bunch of spells. Her MOST powerful spell packs enough of a punch to wipe out the entire universe if she botches it -- basically, it calls on the Slayerverse's equivalent of "God" (the Lord of Nightmares, which despite the name seems to prefer a female aspect, actually). But she can barely control it even using a magical focus (in the first season, she successfully casts it by using Gorun Nova, the Sword of Light, as a focus), and she avoids using it at all costs because of the incredible price of failure with it.
Besides which, it takes time to cast (there's a brief incantation), and it's WAY overkill against anyone who isn't an extremely powerful god-being.
Her second-most powerful spell is the Ragna Blade, which she CAN control reliably but is very draining to her. It also calls on the LoN, but it's short-range (basically a giant black "sword" of uncreation) and also requires an incantation. Again, pointless overkill against Storm, and again, it takes time to cast.
Her third-most powerful spell is the Dragon Slave (originally "Dragon Slayer" because it was first used to kill a dragon in a single shot, it became basterdized to "Dragon Slave" over the years). It's basically a big freaking nuke that can take out a small city (or at least a good chunk of it) or thereabouts. Again, it has a cast time.
Beyond those three, she can cast pretty much everything with just a word and a gesture. Her arsenal includes lightning bolts (Digu Volt), fireballs (Fireball), an air blast of sufficient force to crumble multiple layers of solid stone (Dimilar Wind), a kind of area attack that fires a wide cone of flares (Rune Flare), a simple fire bolt spell (Flare Arrow), umm ... a lot of things, actually. I'd have to compile a list.
She can also cast a flight spell that comes complete with a low-strength shield as part of the spell (Ray Wing) or a simple levitation spell (Levitation), and she occasionally uses a variant of Lighting (a light spell) to create a blinding flash to disorient foes. There's Demona Crystal (creates an icy fog that comes up out of the ground and then suddenly freezes the target solid) and Shadow Snap (toss a dagger at the target's shadow while casting this spell, and it "traps" the shadow ... pinning the target in place). And of course she can create various defensive shields. The most remarkable such shielding I can recall her casting was when she cast a shield sufficient to protect herself from her own Dragon Slave (magically amplified using her demon's blood talismans) cast at point-blank range. She's not the most impressive defensive magic-caster in the series, though (that would be Slyphiel
).
Aside from spells, she's also an accomplished swordfighter (not as good as the best in her world -- who happens to be her boyfriend -- but definitely high on the scale), decidedly acrobatic and exhibits exceptional reflexes and incredible durability.
However ... for all of that, she's still human. Her durability seems to be incredible for a human, but she CAN be injured and even killed (she almost WAS killed in the first season by a direct blast from an extremely powerful sorcerer fused with an evil god, after pushing a friend out of the way of said blast).
Really, I don't think I can give you enough to judge if you haven't seen her in action. She's incredibly powerful, but at the same time, she can be hurt, so it's not like she can just stand there Superman-like and shake off whatever Storm throws at her. I think it'd be interesting.
