Loki can run quite lightly and quite fast, even armored.
She walks up to the pedestal on which Mjolnir rests, and she takes the handle in both hands and lifts. It comes up fluidly. Thor steps back.
It's a day's ride to the last known location of the bear. No one gives them any trouble on the way.
In the event, they do find tracks; finding this bear has never been a problem. Thor suggests that they dismount and lead the horses along the trail; she has never used Mjolnir before and is sure she could not wield it from the back of a terrified horse.
It is much, much larger than anything or anyone Thor has fought before. Its shaggy hide is dotted with the broken ends of old arrows, and scars from the swords or axes of previous opponents. Its paws are each individually bigger than her head. It could bite her in half with a single chomp and not take much trouble about it.
She takes a deep breath, raises Mjolnir, and charges with a yell as thunderclouds fill the sky.
...Loki's spell cannot actually resurrect the dead. And she would need to be conscious to cast it, even if all the difficult work of assembly is over and done. She suddenly regrets this very much, wishes she'd run and told Father, but there's nothing for it now. She circles around, looks for advantageous terrain - they'll need all the help they can get. At least she and Thor can flank it.
She does not expect the bear's head to snap back with a loud crunch, nor for the hammer to keep going, dragging her arm in a full circle until she wrests control from it at the last moment and hauls it up to strike at an incoming paw. Claws screech on metal. Thor lets the hammer's incredible momentum spin her in another circle, and when she comes back around she dips low and swings high and leaps for the bear's face.
The bear does, in fact, start to stand. Mjolnir hits it in the chest. It falls over backward; Thor lands on its chest and smashes its already broken jaw one more time with the hammer.
Well, Loki's a little redundant, although she may have usefully distracted it. That's all right, Thor is okay so far, so is she, that's the point. She drops onto her stomach at the edge of the ledge she's standing on for reach and stability, stabs down at the bear's stomach; she'll be on her feet in a moment if it gets its footing back.
Thor stops hitting it when it stops moving; she jumps off, rolls, comes up facing the bear's corpse, and backs away warily, because with beasts like these you never know.
It remains dead.
She laughs.
"Sorry I didn't leave any for you, sister!"
"I should clean my hammer," she says. "And you, you've blooded your spear, sister!"
"We should camp for the night, I think," she says. "It's too late to make it back home by morning."