Damn good, she started a war with her sister on Charn that lasted, ages and kept her people fighting to the bitter end and when she did ruin it, she retained enough sense to put herself in stasis.
As to her flipping her wig on Earth, I think that had to do with what happened to her when she got pulled through the woods between worlds or whatever it's called where she started to whither and turn into a husk and the experience left her a little retarded.
In Narnia she had hundreds or thousands of years to build up her skills and power again.
She didn't just blanket everything in winter, she stopped time across Narnia for the entirety of her rule, holding the land in a temporal lock, delaying Christmas and Santa and the spring. And no, if I remember right she held the land frozen in time for only a century. She spent the other nine centuries conquering the North and learning magic native to Narnia to better enslave it and then she re-invaded Narnia.
That's when the century long time lock happened.
She should be able to both outwit and out General Ned, as for Tywin, he'd be a good deal more formidable as Jadis got a bit crazy but this is a woman who managed to convince half her planet to go to war with the other half so she could take over and kill her sister. She also built a second, then third empire for herself and spent a hundred years slowly conquering two other regions of Narnia.
She's no Machiavelli, but she's also no slouch.
She'd have trouble controlling her rage playing the game but she shouldn't be inept at it by any means.
As to her strength, she kinda required Aslan to body