... We don't know that she had a Host at that point...
No, you're right. I was conflating another moment.
It's always possible that Hannah was lying, but she claimed (during the duel in Hades' vault) that (as of Changes) she was actually allied with the Order of St.Giles and "fighting the good fight." Part of what turned her to despair & picking up Lasciel's denarius was watching all her new friends and allies get destroyed by the Blood-Curse ritual.
... I don't think Hannah without Lasciel's help would have been stronger in fire than Harry ...
According to Harry, I think she was.
She was known to the Wardens as a dangerous, combat-heavy fire-mage.
Every warden who went after her would have been prepped accordingly.
But she took down
multiple fire-prepped wardens with
just fire-magic.
A big part of what makes Harry so dangerous is that he's got so
many arrows in his quiver. Fire mage, air mage, force/telekinesis mage, potion-maker, vulcanomancy...
necromancy! And serious sideline-talents with earth-magic, water-magic (per WoJ, I don't think we've seen it in the books). Savvy-enough and strong-enough to disrupt the workings of Entropy-mages.
Hannah Ascher is a
MUCH less robust target than Harry Dresden!
And yet -- just with fire-magic -- she was able to take down warden after warden.
Harry could have done it, likely. But not if they knew ahead of time what kind of magic he'd use...