Remember she had about 3 days casting 24h a day - when she was repairing the damage to her mind. The permanent shadow might well be lower but she’s also running that 24h a day more or less and she’s been getting bored and doing things with it (like crochet). I don’t think she’s been respecting the 10 hour a day limit, to be honest. She doesn’t sleep and all she does is study, cast and eat. She was doing 10 hours a day when she had lectures to go to and homework to do.
Power is important, of course it is. But she sliced through Lacer’s control of his spell. I didn’t add that to the list because I’m not sure she quite realised the importance of what she did, but if she can do that, the more power someone puts behind the spell the more dangerous it becomes because of the amount of backlash they have to deal with.
If Lacer tried flooding the area with fire he can’t really do much else. He’d need some form of protection to stop himself suffocating or burning and he wouldn’t be able to track her down.
Meanwhile she just needs to deflect the heat and get out. That’s obviously easier said than done, but she has a much smaller area to focus on and can do other things at the same time - he would be exhausting his strength. Of course, now that she has Legion she could always just escape that way without necessarily letting Lacer know she’s not there anymore, which leads to him having to continue to cast or attempt to divine her location.
I’m not saying that the fact that he was trying to keep her alive was a factor in their duel, or that power isn’t a factor.
I’m just saying that Siobhan, with less than a thousand thaums, is already a difficult and unpredictable opponent who’s taken her first fledgling steps as a free caster.
Siobhan with two thousand or more thaums under her belt and more experience free casting?
She’ll be terrifying.
She’s much more deadly at lower thaumic capacity because of her focus, clarity and intent of her Will. She’s directly faced off against students with higher capacity than her in a 1:1 competition - and in most cases, triumphed. Nunchkin was her only real defeat.
She’s also deadly because she’s capable of splitting her will - allowing multi pronged attack and defence at the same time.
On top of that, she has abilities that are literally out of myth - teleportation, for all intents and purposes. A void shield in progress and capable of being turned to offence. An aberrant that can sense spells literally before they’re cast and help formulate a defence. She’s also absurdly difficult to locate.
She terrified a pair of senior red guard agents with a potion, her shadow and a song. In a real battle that kind of ingenuity is priceless.
Edit: I just read that you said to be safe - which, yes, fair. Stronger shields, stronger spells - these will definitely help you win a fight. But as we’ve seen from Siobhan herself, you’re never really safe in a battle, even if you out power your opponent.